Changed Android app to builtin NativeActivity

What this means: the Android app is now pure C++ with JNI calls to java.
Android 2.3+ is now required unfortunately, but will allow Xperia Play
touchpads and other more exotic accesories to work in the future (not yet).

To clear up any confusion: what was once in NativeApp.cpp has been moved to PPSSPP.cpp, but I did so by copy-pasting content instead of renaming the files, so the merge looks a bit messy.

Minor functionality is still needed, such as: Making the launchBrowser func work.