Use mock window manager for BubbleController in tests

Bubbles are no longer part of the status bar window and
live in their own window now. This resulted in a new
window being created each time setUp step happened in
BubbleControllerTests without ever being removed, resulting
in OOM when running tests :(

This CL fixes it by using a mock window manager in tests.

Test: atest
Fixes: 155805511
Change-Id: Id1940aeb7ee94801fa5669a30a98e2ea9c92126b