Default flexible YUV format to NV21

There could be buffers allocated in flexible YUV format without any
camera usage flags. Default the format for such buffers to NV21.
Fixes CTS tests testYuvImageWriterReaderOperation, testAbandonedSurfaceExceptions

Change-Id: I72693b1d35d2803df16d3588b3428bcf1ab0b3eb
1 file changed