User switcher - Fix broken user addition

Adding users from accounts other than the non-system user was not
functional. Simply use the context to launch the CreateUserActivity in
a new task stack.

Also, address test failures where Dependency.get() was being used by
other tests with the wrong context, creating failures in the user
switcher tests. Remove this call from SystemUIDialog and update it
everywhere.

Fixes: 218989709
Fixes: 218824658
Test: atest KeyguardUserSwitcherAdapterTest UserDetailViewAdapterTest KeyguardSecurityContainerTest UserSwitcherControllerTest InternetDialogTest SystemUIDialogTest
Change-Id: Iaceb3dd2add7d2372f9ec646610c8e147c516e63