c2 vp9, av1: Produce 8 bit output when color format isn't set

When decoding 10-bit clips, if color format isn't set in format
then decoders are expected to produce 8-bit output to maintain
backward compatibility.

Bug: 232472398
Test: atest android.mediav2.cts.CodecDecoderTest
Test: Decode 8-bit clip by not setting color format in configure
 and verify that output color format is 8-bit compatible

Change-Id: Ie28c4e1e82d35ae5b8f20cb775d55fad6ff59b54