Wire user info and use user context for text toasts

Text toasts don't currently support multi-user. This CL prepares toasts
for multi-user by wiring the user information from who requested the
toast (in the form of uid) to ToastUI, where text toasts are actually
rendered.

We go only as far as obtaining a new user-specific context for that user
and using that to construct the view. Actual support will come in future
CLs.

Bug: 151414297
Test: atest android.widget.cts29.ToastTest android.widget.cts.ToastTest
      ToastWindowTest ToastUITest NotificationManagerServiceTest
      LegacyToastTest
Change-Id: I8c92453c6a2b73c31f9a41ca9ff463d194d4f44f