Update PrecompileContext::getPipelineLabel to, optionally, return the uniqueHash.

Android's PipelineCallbackHandler needs this functionality to be able to prepopulate its cache just from serialized keys.

Bug: b/482036727
Bug: b/498213682
Change-Id: I7875de1a77acff13de24528e0f65cdcf35793d63
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1204976
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>