Fixes some issues for embedded Task

1. Don't compute minimum dimensions for embeddedTask in TaskFragment
2. Consider TaskFragment containing ActivityRecord and embedded Task in
   Task#getTopResumed(Pausing)Activity
3. Add support to start Activity on top of existing embedded Task in
   TaskFragment

Test: SplitActivityLifecycleTest
Bug: 192442647

Change-Id: I8c0988e5070053c6a3b336184aa0a9ebd79adfda