Move RecentsImpl task stack listener to background.

- In RecentsTaskLoader, synchronize on anything that might use the
  thumbnail cache
- In RecentsLoadPlan, remove synchronization when preloading/executing
  since that is done in the loader now
- In RecentsImpl, synchronize on anything that might use the dummy stack
  view

Bug: 37550083
Test: Launch Overview and some other tasks, ensure everything still works

Change-Id: I36d04566ebb04296f67a3233730c1dc5bc1569f4