Make IMMS#mSettingsObserver multiuser-aware.

It turns out that IMMS#SettingsObserver has monitored
only main user's the secure settings. As a result, when the
secondary user changes enabled IMEs in the settigs app,
IMMS only updates internal enabled IME lists only when
the user is switched for secondary users.  If a secondary
user enables or disables IMEs at the settings app, such
changes are not correctly notified to IMMS.

This CL addresses above inconsistency by explicitly
specifying the user ID when calling
ContentResolver#registerContentObserver().

This CL also allows dumpsys to contain internal state of
IMMS#mSettingsObserver in case we need to diagnose
problems only with bugreports.

Bug: 19340792
Bug: 19587437
Bug: 21612582
Change-Id: I34b437928c147e9fdbe935f725624cc97c816cb3