VP9: add vpx_codec_get_global_headers() support

This returns the contents of CodecPrivate described in:
https://www.webmproject.org/docs/container/#vp9-codec-feature-metadata-codecprivate

The value for 4:2:0 is 1 (colocated) to match the default given for the
codec parameter string:
https://www.webmproject.org/vp9/mp4/#codecs-parameter-string

Bug: b:332052663
Test: presubmit
Change-Id: Ie50dd8d76e2d7389ac01bf4dbec801f9c8ea0e21
(cherry picked from commit 63b9c2c0e2aeaa2964b7b64d803a198003dcb853)
(cherry picked from commit e44918bd4e83fe4994eb1396a1c2da88862e24d7)
3 files changed