Audio HAL VTS: GetPresentationPosition may return 0 on stop stream

GetPresentationPosition returns the last time a sample was written to
the hardware.
Calling it on a stop stream should return 0 sample written, but the
timestamps of that measure is of little importance.
Thus allow this timestamp to be 0 indicating that the measure was never
actually made.

Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Test: call/play music/record/video...
Bug: 36311550
Change-Id: I3c33b60f98e8cbea269a7739cc1889af932dcff5
Signed-off-by: Kevin Rocard <krocard@google.com>