CtsWindowManagerBackgroundActivityTestCases improvements.

- Refactored assertTaskStack() usage into
  assertTaskStackHasComponents(...) and assertTaskStackIsEmpty(),
  which uses Truth assertions
- Added user id on activity lifecycle events
- Implemented toString() in some POJOs

These changes will make it easier to identify failures.

Test: atest CtsWindowManagerBackgroundActivityTestCases
Bug: 267503734

Change-Id: I59ef39fb63df9ff5b2d254ab0ac33ab836d292c1
4 files changed