Build optimizations to reduce the size of Android shared libraries.

These changes bring the NEON androidEmptyExample library from 15.2MB down to
6.4MB. In conjunction with "ABIS_TO_COMPILE = neon", the Android APK is now
under 3MB, which significantly improves app deployment times.

This resolves issue #3416.
Merge pull request #3421 from nneonneo/issue3416

Build optimizations to reduce the size of Android shared libraries.