Make IMMS.SettingsObserver multi-user aware

With this CL IMMS.SettingsObserver becomes multi-user aware, by being
registered only once for all the users.

The observable behavior should remain to be the same for single user
mode, and for concurrent multi-user mode the new behavior should be
the right one.

Fix: 342027196
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I35253d3964f37f3e13888c0017a86449cf1de8ac