Minor cleanup of Task

- Remove duplicated invocations of allPausedActivitiesComplete.
- Reorder condition in shouldSleepActivities.
  Because isKeyguardGoingAway is much cheaper and usually false.
  While isFocusedRootTaskOnDisplay needs to iterate hierarchy.
- Remove unused methods.

Bug: 163976519
Test: CtsWindowManagerDeviceTestCases
Change-Id: I09c3dd8d33c4bf0fa899214e22ad41b72ff02735