Track volume should be maintained even if track is restored

If track is restored when it set its volume to a non-unity value (such as mute),
the volume was forced to unity again.  Now, the previous volume is restored.

Bug: 18570664
Change-Id: I79f34c6c7d6f535baba813fd3b8ba8e55cac8ad7
stagefright: Clear mNoMoreOutputData flag during seeking

For some 3rd party players, stop won't be called when playing
to the end. If seeking happens after that, we need to clear
this flag to make it be ready for subsequence data flow.

Change-Id: I9b2d18d03225420a45821ba7536cdc91d52b1312
CRs-Fixed: 755988
libmedia: Fix crash seen while running CTS

 * setVolume was called while the track was being torn down during
   MediaPlayerFlakyNetworkTest

Change-Id: Icb965478644909511215c17db3cf2debba85a864