Teach TestableSettingsProvider about special UserHandles

There are a couple UserHandle values that need to be special cased in
order for TestableSettingsProvider to work properly with the ForUser
variants of functions. Without them, they'll treat them as entirely
unique users, when in reality they're notation for the current user,
whomever that may be.

Fixes: 274430330
Test: atest DisplayPowerControllerTest DisplayPowerController2Test
Change-Id: Ic5ca443b16e25cef3264f8884f3fa0154508a8f3