Pass userId through to singleton ContentProviders

System content providers like SettingsProvider run as singleUser but
on receipt of a call make decisions based on the calling user ID.

This is right up until the caller is a system service or a cross-user-
-aware app like Settings, where putStringForUser etc. provide a
workaround for most settings but not for the few files SettingsProvider
exposes.

Change-Id: I90060c9c13e274edd71f8a16ab3a026a58b98e3e