gralloc: Default flexible YUV format to NV21_ZSL

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