vp9: Reorganize the buffer level for cbr mode

vp9: Reorganize the buffer level for cbr mode

Refactor the code with some changes.

Split update into two parts: move the fillup
(with per-frame-bandwidth) before the encoding, and
keep the leaking part (with encoded_frame_size) after
the encoding (postencode).

For SVC with ref_frame_config usage: allow usage of timestamp
delta for the fillup part of buffer, instead of the (average)
framerate passed in via the duration.

Moving the buffer fillup (+per-frame-bandwidth) part to the
pre-encode causes some difference in performance
(since buffer level affects active_worst/QPand frame-dropping),
but the change is observed to be small.
Made small adjustment to active_worst_quality to compensate.

Adjust some thresholds in datarate tests.

Change-Id: I81a5562367034f318cffd451304bc4a34bf02a1d