Encapsulate the pending task events logic.

- Add a new class TaskOrganizerPendingEventsQueue to hold all the pendingEvents
  for a given TaskOrganizer.
- Also made sure that pending events list is cleared when an Organizer
  gets unregsitered or dies.

Fix: 224812558
Test: atest WindowOrganizerTests
Change-Id: Idfac9fc5cd02b5f91324a5cd27639e1c258ce587