stagefright: Avoid querying time from AudioPlayer after audio EOS

If a clip's audio track is shorter than its video track, the system
clock is used as the time source after audio EOS. However AwesomePlayer
continues to query the media time from AudioPlayer within
onVideoLagUpdate. This may take longer than the expected onVideoEvent
interval of 20 ms which can cause delayed or dropped frames.

Additionally, if onVideoEvent is more than 500 ms late, AwesomePlayer
triggers a video-only seek to the incorrect audio EOS media time. Since
the system clock continues to tick, video becomes even later after seek
and seek is triggered again, resulting in a continuous seek.

Stop querying the media time from AudioPlayer after audio EOS to avoid
this scenario.

Change-Id: I5c56a51509f8787d384696c703a9327b0fa5dbac
CRs-Fixed: 739308