ANDROID: Fix a race condition

setSurface is done in a different thread than the one that starts
the scummvm main. The main thread would then wait until the setSurface
thread notifies. The setSurface thread would notify before it actually
calls setSurface, meaning if the thread is preemted before calling
setSurface, initSurface will assert, causing the app to crash.
Merge pull request #432 from klusark/race

ANDROID: Fix a race condition