fix playback position after switching to offload

After switching from offloaded track to PCM track
while paused (e.g. when connecting A2DP), playback
restarts from the beginning of the song when resuming.

Save current position before recreating an AudioPlayer
in AwesomePlayer::play_l() and seek to the saved position before
starting playback.
Also fix a problem where the position is not reported properly
by AudioPlayer if a seek is pending and queried just after start
and before the first buffer is read from the MediaSource.

Bug: 8174034.
Change-Id: I254e65418ff903a9bf2e2111b89a00e2e54876c5