Make animation notifications thread safe.

Fixes: 322266912

A system UI was calling pause() on an animator from a non-UI
thread. Previously, this was safe as long as the listener
list didn't change. Now, there is a possibility of a crash
because the cache is reused with the call.

This CL makes the access to the cache thread safe.

I tried to make a test to validate that this code works, but
it turns out to be extremely difficult to reproduce. The
pausing thread must interrupt between just a few instructions
and even running several hundred thousand times, I could not
reproduce it.

Test: ran existing tests
Change-Id: Ic0affd711ca0330edabd5f7f65f2731646ad8c77