Adjust the quality of boosted frames

Adjust the quality of boosted frames

As the boosted frames, early in key frame interval,
are used as reference by many subsequent boosted frames,
boosted frames that are closer to the reference key frame
should be allocated with more target bits than the rest.
Similarly, the active best quality should be lower for
boosted frames early in the key interval and vice versa.
Hence, the bits allocation and active best quality are varied
based on their temporal position in the key frame interval.

Change-Id: I1362248560d074b9e209657a23ae73dda0b01d52