ImageReader: override the flexible YUV compatible formats

For gralloc HAL v0.1 devices, if the producer buffer format is NV21 or YV12,
the returned flexFormat will be NV21 or YV12, which causes CTS failure
for ImageReader decoder test. This change overrides the NV21 or YV12 image
formats to HAL_PIXEL_FORMAT_YCbCr_420_888 for such case. With this, the
ImageReader will work for the devices with older gralloc HAL implementations
for HAL_PIXEL_FORMAT_YCbCr_420_888 compatible formats.

Bug: 27136665
Change-Id: Ib4722f1f8dc20ad6561088755e4ab9d2e68f1b47