AudioTimestamp: Use current time when data pipeline is empty

Handles change in Q, where we do not return an invalid kernel time
in the extended timestamp when there is a reported error from the HAL.

Test: Photos, File playback, AudioTrackSurroundTest#testPlaySineSweepBytes
Bug: 113294730
Bug: 118506063
Bug: 119895849
Bug: 120631792
Change-Id: I90f2c7c314baa474c3dd3a24042f1d4b8bc0a12b