8/ Updating bubbles to run on shell thread

- Refactor some code to prevent blocked calls when calling to
  and from the Bubbles code when it runs on another thread
- Ensure that ScrimView can be run in separate windows running
  on separate threads

Bug: 161979899
Test: atest WMShellUnitTests
Test: atest SystemUITests
Change-Id: I8eca9e9f644486f73a39f21dc37852be51d94539