UserController refactoring

Addressed comments from the previous cl. Added getters for UserController
fields, direct access is no longer allowed. Moved the following methods:
 - getUserManagerLocked. Became getUserManager() - because locking is not
   necessary, double checked locking will suffice.
 - startUserInForeground /showUserSwitchDialog/sendUserSwitchBroadcastsLocked
 - handleIncomingUser/unsafeConvertIncomingUserLocked/isUserRunningLocked/
   getUsers/getProfileIds

Bug: 24745840
Change-Id: Id5a5cfb9604e08add29bd9a03c8fe5200bc51fef