Add GOP_COMMAND

Add GOP_COMMAND

Send GOP_COMMAND to vp9 for setting gop decisions on the fly.
GOP_COMMAND has three members.
use: use this command to set gop or use vp9's gop decision.
show_frame_count: number of show frames in this gop.
use_alt_ref: use alt ref frame or not.

Move the logic of processing external_arf_indexes_ from
get_gop_coding_frame_num() to GetGopCommand() and
GetCodingFrameNumFromGopMap().

Change-Id: Ic1942c7a4cf6eecdf3507864577688350c7ef0cf