Android: fix some problems with the media player.

- Correctly emit positionChanged signal. One of the problems with
  this was that QMediaPlayer automatically sends periodic
  position updates while playing a media. There's no need to have
  the same logic in the backend.
- Seeking after reaching the end of the media now correctly works

Auto tests included.

Change-Id: I6d5ecbae6e05f94a8aac1a0834cf57427adf219b
Reviewed-by: Christian Stromme <christian.stromme@digia.com>