C2AllocatorGralloc: Fix stride in map()

This is a continuation of the CL with change-id
I1a587544fcf864d4a239252a8a2dc2569161e279.

The code has another typo where "if" should be an "else if",
which causes the stride on mapper2 always set to 0.

bug: 143551816
test:
- play vp9 test clip in the bug for sw renderer on Pixel3
(modify xml to force sw decoding), video should look normal.
- run MediaMetadataRetrieverTest#testThumbnailVP9Hdr with output
file dump, output should look normal.

Change-Id: Ia384dfa1cf37947cd20709ecebacd0158ae3329f