Add account preference controller.

- Add account preference controller and move the handling for
displaying user's accounts from AccountSettings to the
controller.

- Move AccountPreference and RemoveUserFragment from inner class to
standalone class.

- Add AccountRestrictionHelper to handle API that needs to be mocked for
testing.

Bug: 31801423
Test: make RunSettingsRoboTests
Change-Id: I3d16d0b6a8922a8abec7037c52b7a4de2d76eb0d