Return number of samples in NativeApp::audioRender()

This way we can try to avoid writing invalid/zeroed data to the buffer.

For me, this improves audio significantly in e.g. the menu of Senjou no Valkyria 3.  Otherwise it's really delayed and choppy.  Hopefully may fix hrydgard/ppsspp#1883.

`audioTrack.write()` blocks, but only for writing, but `Thread.sleep()` doesn't have great resolution so I just used `Thread.yield()`.  I haven't figured out the right way to profile Android yet, but this sort of change helped perf on iOS.

-[Unknown]