SkFlattenable::Register now performs a sorted insert

In order to maintain the sorted quality of `Entry gEntries`, I updated SkFlattenable::Register to perform a sorted insert with the new element. To avoid possible threading issues, I added an SkSharedMutex.

Change-Id: I14a9001ef2822211bbcbe994b73b4671ae17844d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1252956
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>