Clean up the behavior to support toast shown on Lockscreen.

I7092f2d, Support to show toast over lockscreen for Pixel dialer with
carrier requirement. Text toasts will be rendered by SystemUI instead
of in-app to prevent abuse and snackbars are preferred for brief
messages while the app is in the foreground. Pixel Dialer also
confirms they don’t need the behavior from R. Clean up the code which
is useless.

Bug: 163441727
Test: atest DisplayPolicyTests
Change-Id: Ia6cd9c0ec2ca39825d6d3e74f7852a3ce6b305c4