Make taskbar focusable when folder is open to get IME input

- Remove ActivityContext#supportsIme(), as it's always true now
- Add OnFolderStateChangedListener, which we register when clicking on a taskbar folder icon to set the window focusable/not.
- Also remove Folder.STATE_NONE and instead default to STATE_CLOSED (renamed from STATE_SMALL).

TODO: make sure back button is visible above IME (followup CL)

Test: Open a folder from taskbar while in an app, no animation jump and can change folder name
Bug: 180051157
Change-Id: I7c7847657d462c16677d66b9ffa8b6fe5f164084