Avoid jumps to faulty position after seeks

When seeking multiple times it is possible that some seeks
are discarded in AwesomePlayer, which causes unwanted jumps
to faulty positions.
The reason is that a seek flag is reset twice in AwesomePlayer.
At first when the video seek is completed and then again when the
audio seek is completed. If a new seek is made after the previous
video seek completed but before the previous audio seek completed,
the new seek position is discarded by the previous audio seek
completion.

This fix makes sure that the seek flag is reset only when video
has completed the seek.

Change-Id: I8f8741d4cb8682345f1d1855bbad57c05f4e3c8d
(cherry picked from commit bf979693182b59fc793d5d2683185ecc54246692)