Bump target framework to 20.
Bump target framework to 20.

This is for XamGuard to work correcty as the new Google Play Services
library references APIs from framework 20.

ZhangLeiCharles/mobile,ZhangLeiCharles/mobile,peeedge/mobile,eatskolnikov/mobile,masterrr/mobile,eatskolnikov/mobile,peeedge/mobile,eatskolnikov/mobile,masterrr/mobile