Add tests for toast block

Topic CL gates the background custom toast block on targetSdk, so
adding tests for the all the cases in the 2x2x2 matrix of
(text toast, custom toast) x (bg, fg) x (t < R, t >= R), where t is the
targetSdk of the app.

Bug: 144152069
Test: atest android.widget.cts29.ToastTest android.widget.cts.ToastTest
Change-Id: I0a055f50c7727a84d65bf33dcd192354ecb4b718