gralloc: test lock YCBCR_420_888

Gralloc 4 removes lockYCbCr but clients will still expect similar
functionality to lockYCbCr. This test case ensure that gralloc
will still behave sufficiently similarly when locking
YCBCR_420_888.

Bug: 141631415
Test: VtsHalGraphicsMapperV4_0TargetTest

Change-Id: I60d2c220bab858eabe89a95b225d2816ab7289cc