Fix for SettingsProvider to query for correct user.

insertForUser takes a specified user and attempts to adjust that user's
settings, first looking at their existing settings to determine the difference.
However it was querying the settings for the calling user, rather than for
the user whose settings were being changed.

Also add a test that exercises the fix.

Change-Id: I6ed6fd79154ac1b6e6ab880769ac9081dfff6b80