Android: fix media player's volume.

- Preserve the volume when changing the current media.
- Don't actually set a new volume when the media player is muted.
  Intead, save the value to apply it when setMuted(false) is called.

Change-Id: I829eb280406ea35ff82a0c2638b2d19cf1d90643
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Android: don't start camera preview until the viewfinder is ready.

If no video output is set for a camera, don't try to start the camera
and report an error.
If the video output is not ready, delay starting until it is.

Change-Id: Id08e31a4e795b71ac036a6532e9499ca5670d790
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Improve default audio plugin selection.

When no audio plugin is marked as default, use the first plugin
available.

Change-Id: Ide8db0fe55f43c4881c24505c1e2821b0fc176f6
Reviewed-by: Dyami Caliri <dyami@dragonframe.com>
Reviewed-by: Christian Stromme <christian.stromme@digia.com>