Wrap MTLEvent code in availability attributes.

Allows us to build with a newer SDK and an older deployment target.

Bug: skia:8243
Change-Id: I06dc2737e2661dc81d98c2411b93123642d4a1e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240514
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed