Bug 1075306 - Implement InputWindow (inherits from AppWindow)

- Instances of InputWindow replaces those iframes we were keeping in InputFrameManager.
- InputAppTransitionManager has been eliminated, in favor of already-built AppTransitionController that works closely with AppWindow.
- The coupling of KeyboardManager, InputFrameManager and InputWindows shall be resolved in a following bug where we'll implement InputWindowManager.
- Flow of showing/hiding/reusing/switching input apps & layouts has been streamlined.
We need to clean this in followup; if inputLayoutManager needs to clean up  it should be called from InputWindowManager whenever InputWindowManager.hide/hideIMME is called; that is to say, the logic to hide keyboard should be moved into inputWindowManger.