Bug 1062664 - Don't update MediaSourceReader::mLast{Audio,Video}Time
when seeking.

Any On{Audio,Video}Decoded callbacks triggered before we request a new
sample from a reader after seeking should not be used to compute the
last sample time because they originate from the reader we're switching
away from and cause us to switch back to it after seeking.
Bug 1064160 - Move NotifyGotData calls to TrackBuffer so they are closer to the state being waited/notified on.

Rename NotifyGotData to slightly clearer NotifyTimeRangesChanged.  Move
WaitForData from MediaSourceDecoder's public API to MediaSourceReader's
private API and hoist the wait loop logic inside it.