Building ffmpeg executable on Android

Current versions of build scripts do not create ffmpeg and ffprobe binaries but it is possible to generate them by applying the following changes.

In build/android-ffmpeg.sh

1. Delete --disable-programs line

2. Change --disable-static line to --enable-static

3. Change --enable-shared line to --disable-shared

In build/android-common.sh

4. Replace all occurrences of libc++_shared with libc++_static

After applying these changes, android.sh will create ffmpeg and ffprobe binaries under the prebuilt/android-<arch>/ffmpeg/bin path.