codec2: use flexible YCbCr 420 888 format for GraphicBuffer

Some platforms only support NV12 instead of YV12. Use flexible format to
contain both.

Bug: 73261432
Test: adb shell setprop debug.stagefright.ccodec yes
Test: adb shell setprop debug.stagefright.ccodec_v4l2 yes
Test: adb shell setprop debug.stagefright.omx_default_rank 1000
Test: adb shell killall mediaserver
Test: cts-tradefed run cts -m CtsMediaTestCases -t android.media.cts.MediaCodecTest
Change-Id: I6386afe88793fe40b9eba0b17baa650999e99fa0