Fix stride number in C2AllocationGralloc::map

The code has a 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: I1a587544fcf864d4a239252a8a2dc2569161e279