Only call findImage when needed

Reverse the performance regression caused by genereating
the mask earlier in the calling sequence. This caused the
cache to be purged too frequently in the emoji fling test.

Fixes android bug 131096308

Change-Id: I727481085f85626ae119cab316b788c65701b8f6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209804
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
7 files changed