Update SET_ACTIVITY_WATCHER protection to signature|system

    <permission android:name="android.permission.SET_ACTIVITY_WATCHER"

        android:label="@string/permlab_runSetActivityWatcher"

        android:description="@string/permdesc_runSetActivityWatcher"

        android:protectionLevel="signature" />

        android:protectionLevel="signature|system" />

    <!-- Allows an application to call the activity manager shutdown() API

         to put the higher-level system there into a shutdown state.