[ndk] Fixed sample SanAngeles to properly pause/resume

Previously re-launch of background SanAngeles app shows black or
freeze frame

Two fixes:
1. Not to reinitialize timer each time when DemoRenderer is created
2. Stop advancing internal clock when onPause(), and start when
   onResume()

Change-Id: I6bef9d7c462cd4d504e76099a4d28c8618043f46
2 files changed