Decouple WmTests and ActivityManagerService

- Speed up initialization of thousands of test methods.
- Reduce taking care for unrelated dependencies.
- Remove a few unused fields.

Bug: 159102753
Test: ActivityTaskSupervisorTests ActivityManagerServiceTest
      RootWindowContainerTests

Change-Id: I1a62e7e9bed2dece5573f47e06f5ba9e8fd3b2a3