Fix memory leak

Client was holding onto the finished callback in the
RemoteAnimationRunner, and the server was holding on to the runner
via the finish callback. Cycle! There is no GC for cross-process
binder, so we have to make sure to properly free it.

Test: go/wm-smoke
Test: Open/close apps, take hprof, make sure no leaks

Change-Id: I680953212bee8841c04c2909a4cb82dfac3c2754
Fixes: 72834182