libavcenc: Fix B frame incomplete subgop issue

The encoder doesn't return remaining frames if last subgop is
incomplete. This problem is solved by swapping last NULL P frame
(as subgop is incomplete last P frame will be NULL) with the
last valid (non- NULL) B frame. Complete subgop swapping is not done.

Also update MAX_NUM_BFRAMES to 8 and added a separate macro to
define maximum number of pictures in input queue

Bug: 132063959
Test: run cts -m CtsMediaTestCases \
 -t android.media.cts.VideoEncoderTest
Change-Id: I9625c5f5f64ec60032c44dad54f735991921d6dd