Fix various problems with NativeMediaTest CTS

This test had several minor and major problems:
- one too many slashes in the file:////path/to/file URI
- didn't actually play anything, because it used path-based playback instead
  of callback-based playback
- didn't fail when the test files were missing or inaccessible
- failed when the screen was off

b/6971102

Change-Id: I30324677ec2ddf2746c23e52cf97a4bbbcdfb949