Switch to controling if toast rate limiting is enabled using TestApi.

We tried to use compat changeid to turn rate limiting on/off in CTS
tests but it doesn't work on user builds (see bug). We're therefore
migrating to using a new TestApi guarded by a new permission that we
give to the shell.

Test: atest android.widget.cts.ToastTest
Bug: 175720818
Change-Id: I2ef147ed6449333fc6b4273eca82bce533cb13f3