Hook up custom activity-level animation callbacks to shell

Activity->activity transition animations can be customized by
the client app. The animation part was already hooked up to shell
transitions, but the start/finish callbacks were not. This hooks
them up.

Bug: 194233756
Test: atest WindowUntrustedTouchTest
Change-Id: I2ee6f6e54bf1ebd0f1cbb73cdf79bc25eb0247db