Extract APK assets from native code

Asset extraction on Java side was godawful slow for some reason.
To avoid this, I've written a JNI class which does this in native code.

This improved performance over *300x times*, so asset extraction time is basically unnoticable now.

Since we have to build more than one lib now, ndk-build from phoenix/jni, which will build libapk-extract and libretroarch-activity. For some reason, ndk-build will sometimes clean out libretro cores, so be careful.

Dunno what happened to appcompat .R file. Might be bad, might not. :V