Add Callback API to Toast

In preparation for moving text toasts to system UI, we need a new way of
telling if a toast is being displayed or not since the view won't live
in the app's process. This is needed for our CTS tests and good to have
for developers so we don't take the functionality away from them, so
making it a public API.

Also updated tests in topic CL.

Bug: 128611929
Bug: 144754526
Test: atest android.widget.cts.ToastTest
Change-Id: I41ff1ed11d5af66e415c772c2e5f611a3d1b17fc