Reduce some flaky cases of WmTests

1. DisplayPolicy -> ScreenshotHelper register real broadcast
   receiver because there are other overloading
   registerReceiver methods that are not mocked.
   If the number exceeds 1000, it will crash.
2. The transition logger may access mocked objects on another
   thread, which will crash the test.
3. Wait for activity operations to complete before leaving
   test that releases shell permission. Otherwise the crash
   (no INTERNAL_SYSTEM_WINDOW) on main thread will stop the test.

Bug: 254268165
Test: WmTests
Change-Id: Ie2a7ac74724ff38245641f43338010adb5ed6ea8