SMS mirroring: Add placeholder summary.

Prevents the summary text from shifting once the final text is
loaded, if OEM specific implementations do an async load of
the summary text.

Bug: 37546615

Test: Verified the placeholder works.
Change-Id: Ia16fe53dbf7a8daf96b9fa955ef9a02fa1eb784f

res/xml/connected_devices.xml

@@ -42,6 +42,7 @@
         android:key="sms_mirroring"
         android:title="@string/sms_mirroring_pref"
         android:icon="@drawable/ic_compare_arrows_24dp"
+        android:summary="@string/summary_placeholder"
         android:order="-3"/>
 
     <Preference