Migrate to new UserTracker for currentUser

This is a partial migration, mostly in classes I'm familiar with. This
means that the current user is meant to be retrieved async as part of
initialization or user action, with the execption of
PrivacyItemController that uses the new listener.

Test: atest SystemUITests
Test: manual
Bug: 163579262
Change-Id: I32b5a47194fa0f9607df19f210f30a0e416f7464