Fix a couple shell unit test regressions

- Ensure we only check adding callbacks for the pip controller (and not
  other dependencies using the same ShellInit class)
- Ensure pip bounds state has non-null current/min/max bounds
- Ensure callbacks posted to the main executor are run in shell
  shell controller tests
- Ensure transition changes have a non-null surface (since it's copied)
- Bump up timeout for running test code (it can exceed 20ms), and ensure
  we release any virtual displays created during the test
- Update the test to the new background layer constant previously added

Bug: 262783148
Test: atest WMShellUnitTests

Change-Id: Ib57e1c0b7c32b44953dd33d2fb09a8ecc00f9c0e