gralloc4-vts: fix Lock_YCBCR_420_888 test

Updates Cb and Cr plane indexing. The existing code seems to use
chromaStep as a sub-sampling factor and seems to assume that the
underlying format is tri-planar.

Updates Cb and Cr value checking. The existing code would write
values into the Cb and Cr planes using a value derived from
even-x-value and odd-y-value full-image-coordinates (e.g. (0,1))
but then check against the values in the Cb and Cr planes using
a value derived from even-x-value and even-y-value coordinates
(e.g. (0,0)).

Updates y-plane sample increment check to confirm that it is
multiple of 8. I don't see any requirements stating this needs
to be 32 bits.

Bug: b/146515640
Test: VtsHalGraphicsMapperV4_0Target
Change-Id: Ia9e496ae43b2d1ac9ea8d57a4d4fc55f0be7b2b6