audio: do not update anchor time before opening of audio sink

 In onNewAudioMediaTime mAnchorTimeMediaUs is getting updated
 even if the mediaClock's anchor time is not updated in case of
 Audio sink is not started, this causes video frame drops as
 video checks mAnchorTimeMediaUs to decide media clock is
 initialized or not.

 Make sure mAnchorTimeMediaUs is not updated if mediaClocks's
 anchor time is not updated in onNewAudioMediaTime

 CRs-fixed: 1037592

Change-Id: Icfd741bf8c14889e8ba525de6908815579e9de20