Streamline fillTaskInfo a bit

- Reduce duplicated iteration to traverse activity in task.
- The condition in supportsSplitScreenWindowingModeInDisplayArea
  includes supportsMultiWindowInDisplayArea, so the result
  can reuse to avoid duplicated invocation.
- Move the helper class to Supervisor because it doesn't keep
  state. It only needs one instance.
These reduce ~10% execution time.

The condition INITIALIZING in helper class is removed because
other fields of task info refer from top-running activity,
which doesn't exclude the state. To avoid confusing when
reading the inconsistent criteria.

Bug: 163976519
Test: WindowManagerServiceTests

Change-Id: I3285212ed726d2d2e60c8fe7caa2e21ff1735cbd