[Android] make_apk: Simplify MakeVersionCode().
[Android] make_apk: Simplify MakeVersionCode().

Several small changes that make the function more readable:
- Use a dict to get the ABI version we want in the version code instead
  of an ad-hoc succession of `if' clauses.
- Improve the function's docstring and move the comment above it to its
  body.
- Make the app_versionCodeBase error message shorter.

BUG=XWALK-3473

darktears/crosswalk,lincsoon/crosswalk,baleboy/crosswalk,zeropool/crosswalk,alex-zhang/crosswalk,darktears/crosswalk,bestwpw/crosswalk,Pluto-tv/crosswalk,lincsoon/crosswalk,darktears/crosswalk,Pluto-tv/crosswalk,Bysmyyr/crosswalk,darktears/crosswalk,stonegithubs/crosswalk,rakuco/crosswalk,hgl888/crosswalk-efl,hgl888/crosswalk,chuan9/crosswalk,minggangw/crosswalk,RafuCater/crosswalk,jondwillis/crosswalk,dreamsxin/crosswalk,bestwpw/crosswalk,marcuspridham/crosswalk,lincsoon/crosswalk,pk-sam/crosswalk,xzhan96/crosswalk,pk-sam/crosswalk,baleboy/crosswalk,jpike88/crosswalk,tomatell/crosswalk,weiyirong/crosswalk-1,marcuspridham/crosswalk,ZhengXinCN/crosswalk,Bysmyyr/crosswalk,chuan9/crosswalk,zliang7/crosswalk,marcuspridham/crosswalk,jondong/crosswalk,crosswalk-project/crosswalk,crosswalk-project/crosswalk,weiyirong/crosswalk-1,amaniak/crosswalk,minggangw/crosswalk,jondong/crosswalk,ZhengXinCN/crosswalk,XiaosongWei/crosswalk,Pluto-tv/crosswalk,PeterWangIntel/crosswalk,PeterWangIntel/crosswalk,heke123/crosswalk,Bysmyyr/crosswalk,xzhan96/crosswalk,ZhengXinCN/crosswalk,XiaosongWei/crosswalk,axinging/crosswalk,xzhan96/crosswalk,pk-sam/crosswalk,PeterWangIntel/crosswalk,jondong/crosswalk,DonnaWuDongxia/crosswalk,zeropool/crosswalk,rakuco/crosswalk,bestwpw/crosswalk,crosswalk-project/crosswalk,xzhan96/crosswalk,jondong/crosswalk,tomatell/crosswalk,chuan9/crosswalk,Bysmyyr/crosswalk,siovene/crosswalk,RafuCater/crosswalk,RafuCater/crosswalk,stonegithubs/crosswalk,lincsoon/crosswalk,DonnaWuDongxia/crosswalk,weiyirong/crosswalk-1,siovene/crosswalk,XiaosongWei/crosswalk,minggangw/crosswalk,tedshroyer/crosswalk,pk-sam/crosswalk,RafuCater/crosswalk,crosswalk-project/crosswalk-efl,tomatell/crosswalk,axinging/crosswalk,weiyirong/crosswalk-1,siovene/crosswalk,amaniak/crosswalk,axinging/crosswalk,zliang7/crosswalk,heke123/crosswalk,baleboy/crosswalk,zeropool/crosswalk,Bysmyyr/crosswalk,dreamsxin/crosswalk,marcuspridham/crosswalk,tomatell/crosswalk,PeterWangIntel/crosswalk,darktears/crosswalk,Bysmyyr/crosswalk,crosswalk-project/crosswalk,darktears/crosswalk,weiyirong/crosswalk-1,marcuspridham/crosswalk,PeterWangIntel/crosswalk,bestwpw/crosswalk,hgl888/crosswalk,bestwpw/crosswalk,bestwpw/crosswalk,pk-sam/crosswalk,dreamsxin/crosswalk,tomatell/crosswalk,ZhengXinCN/crosswalk,rakuco/crosswalk,hgl888/crosswalk-efl,axinging/crosswalk,amaniak/crosswalk,tedshroyer/crosswalk,minggangw/crosswalk,xzhan96/crosswalk,tomatell/crosswalk,pk-sam/crosswalk,chuan9/crosswalk,jondong/crosswalk,heke123/crosswalk,tomatell/crosswalk,chuan9/crosswalk,heke123/crosswalk,axinging/crosswalk,hgl888/crosswalk,zliang7/crosswalk,zliang7/crosswalk,rakuco/crosswalk,jondwillis/crosswalk,hgl888/crosswalk,baleboy/crosswalk,alex-zhang/crosswalk,rakuco/crosswalk,Pluto-tv/crosswalk,RafuCater/crosswalk,siovene/crosswalk,chuan9/crosswalk,tedshroyer/crosswalk,jondwillis/crosswalk,DonnaWuDongxia/crosswalk,minggangw/crosswalk,marcuspridham/crosswalk,jondong/crosswalk,crosswalk-project/crosswalk-efl,alex-zhang/crosswalk,crosswalk-project/crosswalk-efl,heke123/crosswalk,hgl888/crosswalk-efl,tedshroyer/crosswalk,rakuco/crosswalk,stonegithubs/crosswalk,alex-zhang/crosswalk,weiyirong/crosswalk-1,RafuCater/crosswalk,alex-zhang/crosswalk,amaniak/crosswalk,baleboy/crosswalk,crosswalk-project/crosswalk-efl,amaniak/crosswalk,siovene/crosswalk,crosswalk-project/crosswalk-efl,RafuCater/crosswalk,hgl888/crosswalk,darktears/crosswalk,crosswalk-project/crosswalk,xzhan96/crosswalk,XiaosongWei/crosswalk,zeropool/crosswalk,heke123/crosswalk,ZhengXinCN/crosswalk,jpike88/crosswalk,ZhengXinCN/crosswalk,zliang7/crosswalk,PeterWangIntel/crosswalk,amaniak/crosswalk,jondong/crosswalk,weiyirong/crosswalk-1,lincsoon/crosswalk,Pluto-tv/crosswalk,Bysmyyr/crosswalk,DonnaWuDongxia/crosswalk,jondwillis/crosswalk,dreamsxin/crosswalk,jpike88/crosswalk,tedshroyer/crosswalk,hgl888/crosswalk,bestwpw/crosswalk,zliang7/crosswalk,jondong/crosswalk,hgl888/crosswalk,darktears/crosswalk,zeropool/crosswalk,Bysmyyr/crosswalk,zeropool/crosswalk,jpike88/crosswalk,amaniak/crosswalk,dreamsxin/crosswalk,hgl888/crosswalk-efl,XiaosongWei/crosswalk,stonegithubs/crosswalk,stonegithubs/crosswalk,Pluto-tv/crosswalk,zliang7/crosswalk,crosswalk-project/crosswalk-efl,heke123/crosswalk,tedshroyer/crosswalk,xzhan96/crosswalk,siovene/crosswalk,crosswalk-project/crosswalk-efl,siovene/crosswalk,lincsoon/crosswalk,baleboy/crosswalk,Pluto-tv/crosswalk,dreamsxin/crosswalk,minggangw/crosswalk,crosswalk-project/crosswalk,zeropool/crosswalk,hgl888/crosswalk,rakuco/crosswalk,stonegithubs/crosswalk,baleboy/crosswalk,DonnaWuDongxia/crosswalk,axinging/crosswalk,DonnaWuDongxia/crosswalk,marcuspridham/crosswalk,jpike88/crosswalk,minggangw/crosswalk,crosswalk-project/crosswalk,axinging/crosswalk,PeterWangIntel/crosswalk,XiaosongWei/crosswalk,crosswalk-project/crosswalk,hgl888/crosswalk-efl,lincsoon/crosswalk,rakuco/crosswalk,chuan9/crosswalk,jondwillis/crosswalk,tedshroyer/crosswalk,hgl888/crosswalk-efl,jpike88/crosswalk,stonegithubs/crosswalk,alex-zhang/crosswalk,minggangw/crosswalk,lincsoon/crosswalk,jondwillis/crosswalk,dreamsxin/crosswalk,pk-sam/crosswalk,baleboy/crosswalk,XiaosongWei/crosswalk,ZhengXinCN/crosswalk,marcuspridham/crosswalk,jpike88/crosswalk,xzhan96/crosswalk,alex-zhang/crosswalk,hgl888/crosswalk-efl,DonnaWuDongxia/crosswalk,heke123/crosswalk,jondwillis/crosswalk,zliang7/crosswalk