[bmp] Fix swizzler initialization

We only decode via an intermediate color xform buffer when
xformOnDecode() is true (see decodeRows).

The swizzler selection should use the same heuristic to match the actual
dest buffer type.

Cherrypick of https://skia-review.googlesource.com/c/skia/+/937497

Bug: 388480622
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c58deb210e62cf57de91b0eb8844b782fc774135)
Merged-In: Iaf240852a9bcf0be02cda84f0e44576da8100dec
Change-Id: Iaf240852a9bcf0be02cda84f0e44576da8100dec
3 files changed