Fixed all Accessibility lint warnings in Settings app.

Change-Id: Ida98444f8942d3618d84c0a6f78cc3653ea4ad24
diff --git a/res/layout/setup_preference.xml b/res/layout/setup_preference.xml
index 93eb9df..f90daa4 100644
--- a/res/layout/setup_preference.xml
+++ b/res/layout/setup_preference.xml
@@ -52,7 +52,8 @@
                 android:layout_marginLeft="8dip"
                 android:layout_marginBottom="8dip"
                 android:src="@drawable/ic_menu_add"
-                android:background="?android:attr/actionBarItemBackground" />
+                android:background="?android:attr/actionBarItemBackground"
+                android:contentDescription="@string/wifi_add_network" />
 
             <ImageButton
                 android:id="@+id/wps_push"
@@ -62,7 +63,8 @@
                 android:layout_marginRight="8dip"
                 android:layout_marginBottom="8dip"
                 android:src="@drawable/ic_wps"
-                android:background="?android:attr/actionBarItemBackground" />
+                android:background="?android:attr/actionBarItemBackground"
+                android:contentDescription="@string/wifi_menu_wps_pbc" />
 
         </LinearLayout>