Make WindowOrganizer and TaskOrganizer non-static

This is to allow unit test against TaskOrganizer, WindowOrganizer for
interations between Shell and WM core.

To test on WM side
```
when(mWindowOrganizer.getWindowOrganizerController())
    .thenReturn(mockedWindowOrganizerController);
when(mockedWindowOrganizerController.getTaskOrganizerController())
    .thenReturn(mockedTaskOrganizerController)

doSomeTest();

verify(mockedWindowOrganizerController).somethingHappended();
verify(mockedTaskOrganizerController).somethingHappended();
```

To test on Shell side
```
ShellTaskOrganizer organizer = new ShellTaskOrganizer(
        mockedTaskOrganizerController)

doSomeTest()

verify(mockedTaskOrganizerController).somethingHappended();
```

Removed also
- Singleton pattern in TaskOrganizer and DisplayAreaOrganizer as it's
  redundant to what's in WindowOrganizer
- static getController methods in both TaskOrganizer and
  DisplayAreaOrganizer as we prefer non-static for testability
- static methods in TaskOrganizer in general
- static methods in WindowManagerProxy in general

Bug: 161711455
Bug: 149338177
Test: make sure everything still works
Test: atest WmTests:WindowOrganizerTests \
            WmTests:DisplayAreaOrganizerTest
Change-Id: I978d20c7b87e73d0f6d22f7ab08188e4d12d745c