Android.mk: use -fvisibility=hidden
Android.mk: use -fvisibility=hidden

brings the final libwebp.so size down 16/20K with arm64/armv7 builds
using ndk-r13

Change-Id: I20d8aba61d6b692b0fc32f4b271e2f9872f03c28

webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp