Fix bug 3004701 do not prefetch during Realize.

Fix bug 3004701 do not prefetch during Realize.

Removed unused Android object states in enum, not used in
 play state machine.
Made SfPlayer responsible for the creation of its
 AudioTrack once it knows the sample rate and number of
 channels of the content to play once it's done
 preparing.
Added notification from SfPlayer to signal when it's
 done preparing. This informs of the audio characteristics
 and kicks off the content prefetching.
In the AudioPlayer realize:
 - do not start the prefetching, do it when the player
  is set to PAUSED
 - don not create the AudioTrack, do it when SfPlayer
  is done preparing.

Change-Id: Ic345040e71cef0e5953ea47366930d1dfbc5e761