Implement peek mode state tracking in MediaCodec

This commit adds a new PeekMode type and mPeekMode member to
MediaCodec to track the current state of the video peek flow.

Bug: 157501309
Test: Currently untested, full CTS coverage coming soon
CTS-Coverage-Bug: 157501309
Change-Id: If8bc83f93d5cf12c1138c467e3152eafc3e92015