Add Toast to show notification importance reset is completed.

Test: make RunSettingsRoboTests, manual test on device
Bug: 172467473
Change-Id: I5f024c82fedf4dc596dcece9ea2f2444f9d86ad5

res/values/strings.xml

@@ -12436,4 +12436,7 @@
 
     <!-- Summary for preference when Bedtime mode is on [CHAR LIMIT=NONE] -->
     <string name="aware_summary_when_bedtime_on">Unavailable because bedtime mode is on</string>
+
+    <!-- Toast message to inform reset notification importance is completed [CHAR LIMIT=NONE] -->
+    <string name="reset_importance_completed">Reset notification importance completed.</string>
 </resources>

src/com/android/settings/notification/ImportanceResetPreferenceController.java

@@ -19,6 +19,7 @@ package com.android.settings.notification;
 import android.content.Context;
 import android.view.View;
 import android.widget.Button;
+import android.widget.Toast;
 
 import androidx.preference.Preference;
 
@@ -57,6 +58,8 @@ public class ImportanceResetPreferenceController extends BasePreferenceControlle
     @Override
     public void onClick(View v) {
         mBackend.resetNotificationImportance();
+        Toast.makeText(mContext, R.string.reset_importance_completed, Toast.LENGTH_SHORT)
+                            .show();
     }
 
     @Override