media: add CBR_FD video encoding bitrate mode

Constant bitrate with frame drops allows a higher quality constant bitrate
encoding for high complexity portion of the input.

In this mode some frames may get dropped (not encoded) to allow neighboring
frames to use more bits while still staying at the constant bitrate.

The exact number of dropping behavior is undefined.

Bug: 183147292
Change-Id: I96d625f958a2a301b11d41f1c4cc12e680d68d68