better playback time tracking in nuplayer

Fixes problem where playback time was being cleared as part of
a reset before media.metrics stats were logged.
Added code so that getMetrics() calls will return proper in-progress
playback time (previously, playback time was added to the metrics data
only at the end).
Added initialization of timestamp tracking pauses for rebuffering.

Bug: 74008613
Test: dumpsys media.metrics observation