Add a reset to rc tracking for dropped frames

VP9/DatarateTestVP9Large.ChangingDropFrameThresh/[34] fails post the
merge of commit#ffa06b37. This commit adds reset of rc tracking info
when frame is dropped, and fixes the causes of the bad interaction
between the tests and the previous commit.

Change-Id: I848acfd9fcb336359662274325190f94aac76eae