Fix looping HTML5 Audio.

When an <audio> tag has the loop=true attribute,
Webkit tells us to seek back to 0 before we are supposed
to have stopped the stream. But by the time that the message
gets back to the Android MediaPlayer java side, it's already
stopped. So after seeking, we play() again if the player is in
the COMPLETED state. Change the code to do this and handle
the case that we call play on a COMPLETED stream
(resetting internal state, etc).

Note that this has the side effect that we will start playing
the stream after any seek on a COMPLETED stream - e.g. dragging
the slider thumb on the progress track after the stream is
finished.

Bug: 5461143
Change-Id: I6cf4d46d9a1985caf9f9ab85dbcf65535c8dcd77