[amlogic-video] VP9 HW decode works again

While we're not currently using HW decode for VP9, with local diffs it
works, or rather, did work, until my recent sysmem plumbing.  With this
change, only the previous set of local diffs are required for VP9 HW
decode to work locally.

Re. Testability-Review, currently the changed code is not covered in CQ
because of the local diffs required for VP9 HW decode (feel free to ping
me oob re. why), and because no amlogic-based HW target is auto-included
in CQ.  This situation should improve fairly soon.

This CL also adds use_vp9_decoder_test which is analogous to
use_h264_decoder_test (and shares much of the code), but currently
neither is on CQ.

Tested: use_vp9_decoder_test added, use_media_decoder --vp9 (with local changes for HW vp9 decode)
Change-Id: I43a92fefbd52cd1496c026ac3e0e4144be6fdee6