Make VIMS.SettingsObserver multiuser-aware.

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

In the case of VoiceInteractionManagerService, monitoring
changes from all the users is OK because event handlers will
read settins from the current user anyway.

Bug: 21766395
Change-Id: I357fe2a7e2a5645355e696c6fbaae762f54ca029