Gets the userId from Context for AccessibilityManager in ToastPresenter.

- In Android Auto, android.widget.cts.ToastTest#testShow_whenTextToast_sendsAccessibilityEvent failed is failed, since AccessibilityManager in ToastPresenter is initialized with the wrong userId.
- ToastUI creates the Context from the uid of the received message, so we can get the correct userId from the Context.

Bug: 154644563
Test: atest android.widget.cts29.ToastTest android.widget.cts.ToastTest
      android.server.wm.ToastWindowTest ToastUITest
      NotificationManagerServiceTest LegacyToastTest
Change-Id: I138620a0a52e65429157719f588c4be56b075368