Track current user without calling into AMS.

From a layering point-of-view, StorageManager is below ActivityManager
and it shouldn't be calling upwards, since that risks locking
inversions.  Instead, track the current user through the existing
SystemService callbacks.

Bug: 28598527
Change-Id: I02d3277eb26288d3b4e1897f4c9318a6288c9551