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
diff --git a/packages/Shell/AndroidManifest.xml b/packages/Shell/AndroidManifest.xml
index f83f670..e572aa4 100644
--- a/packages/Shell/AndroidManifest.xml
+++ b/packages/Shell/AndroidManifest.xml
@@ -378,6 +378,9 @@
     <uses-permission android:name="android.permission.WIFI_ACCESS_COEX_UNSAFE_CHANNELS" />
     <uses-permission android:name="android.permission.WIFI_UPDATE_COEX_UNSAFE_CHANNELS" />
 
+    <!-- Permission required for CTS tests to enable/disable rate limiting toasts. -->
+    <uses-permission android:name="android.permission.MANAGE_TOAST_RATE_LIMITING" />
+
     <application android:label="@string/app_label"
                 android:theme="@android:style/Theme.DeviceDefault.DayNight"
                 android:defaultToDeviceProtectedStorage="true"