libstagefright: avoid audio source to start before audio player after seek

If audio source is started before audio player during seek, audio source
will start allowing the audio data from the beginning (as seekTime for
Audio is maintained in AudioPlayer and is set once we start the audio player)
Once AudioPlayer is started, it sets the seektime and the data starts from seektime.

This change avoids audio source to start before starting audio player.

CRs-Fixed: 402140

Change-Id: Iad42a5235766478740966788cb9e1dd853104b57