Fix restoring animator_duration_scale back to default

WindowManagerService.setAnimationScale() sends an asynchronous message to
Handler to persist a setting value, but WindowManagerServiceRule cleans up
all remaining messages from Handler when a test finishes.  Thus there may be
a chance that global animator_duration_scale setting is left unchanged. This
CL eliminates removing pending messages and makes them finish instead.

Fixes: 121229219
Test: Pass all presubmit tests for Window manager.
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm
  $ adb shell settings get global animator_duration_scale
  1.0

Change-Id: I97a920cd221d9e0ed3f5cbf2172d9188b39d0ca6