NuPlayer: Process time discontinuity separately for audio & video

Whenever NuPlayerDecoder signals a time discontinuity to the
renderer, both audio and video are affected. However, since there
are two decoder instances (one for audio & one for video), this
incorrectly signals two time discontinuities for each track which
can lead to incorrect anchor times.

Make the behavior of signalTimeDiscontinuity track-dependent to
resolve this. Reset the anchor times only for audio time discontinuity
and reset the video lateness only for video time discontinuity.

CRs-Fixed: 809904
Change-Id: I4f992f1901941daae251162f02b8044c51b1cee3