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Changing rendering mode is not synchronous, so provide a callback for
when an animation can start safely.

Add a callback aidl that that just passes an ObjectWrapper, which
provides no type safety.

Add Callback and ListenableResult generic types in the client lib.

Update the animation demo to use new API. Note now there is flickers
since mode switching is not visually seamless yet. Add a few tests.

Change-Id: I903f0b5805aee03bbf15e67bc6314b1e6f0fdc85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814758Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699535}