Add the mutability flag for SettingsSliceProvider

Starting Android S, all PendingIntent should have the mutability flag
assigned to prevent from vulnerability.

Fixes: 172811605
Fixes: 172206649
Test: 1. Play on the slices in Settings search.
2. Make a permission request intent of Settings wifi slice in the
slice-viewer app, clicking on it and make sure it gets redirect to
Settings without crashes.

Change-Id: I86f915bc062a6e632b5ca9c74e232db1036f08de

tests/robotests/src/com/android/settings/slices/SettingsSliceProviderTest.java

@@ -565,7 +565,8 @@ public class SettingsSliceProviderTest {
         final Intent settingsIntent = new Intent(Settings.ACTION_SETTINGS)
                 .setPackage(Utils.SETTINGS_PACKAGE_NAME);
         PendingIntent settingsPendingIntent =
-                PendingIntent.getActivity(mContext, 0, settingsIntent, 0);
+                PendingIntent.getActivity(mContext, 0, settingsIntent,
+                        PendingIntent.FLAG_IMMUTABLE);
 
         assertThat(pendingIntent).isEqualTo(settingsPendingIntent);
     }