Prepare for frame parallel decoding, the reference count buffers
need to be protected by mutex. Move vp9_thread.* to common
folder so that those buffers could use cross-platform mutex
from vp9_thread.*.
(cherry picked from commit 337e8015c9deaf8ab7e8d0c3c132160a77dd1590)
Change-Id: I0587a08447925f4554d7788686a31483c2ae3f37
