Make WindowManagerService.SettingsObserver multiuser-aware.

WindowManagerService.SettingsObserver has been used to monitor
per-user settings hence we have to take care of multiuser
scenario.

So far it's acceptable to monitor settings changes for all the
users because following event handlers are already
multiuser-aware.
- WindowManagerService#updateShowImeWithHardKeyboard
- WindowManagerService#updateCircularDisplayMaskIfNeeded

Bug: 21766030
Change-Id: I1d93095965c852ff90d8c7cc031c4755856a7e41