MediaPlayerService: Reset the bytes written in AudioSink stop and flush

When playing in non-offloaded mode with gapless playback it is
necessary to reset AudioSink's bytes written on stop and flush
(e.g. seeking) so the frames written can be correctly retrieved
for the subsequent song in NuPlayer::Renderer::onAudioSinkChanged().

Without this change, seeks performed during playback will cause
a time position offset of the subsequent song in Play Music.

Bug: 21198655
Change-Id: I0fb6cc94b5a0ff106dc8a71571ed85d75fb00660