Fix AbrController's reference to StreamProcessor lost when switching period

[robertbryer]

When a StreamProcessor for a first period is torn down, ABR stops working for the second period. The second period is fixed at whatever quality it's on when this occurs.

This is because abrController.unRegisterStreamType is called, but only after it's been set to the new period. The result is that the second period doesn't have a StreamProcessor and therefore checkPlaybackQuality doesn't get called. The solution here is not to call when keepBuffers is true, the StreamProcessor has already been overwritten.

[dsilhavy]

Thanks @robertbryer very good finding. The bug described here occurs because of the preload functionality. If no preload is performed before the period switch this fix is not required.