Fragmentize WifiSettings.

- Add button bar feature toward SettingsPreferenceFragment,
  which has existed in PreferenceActivity and has been used
  (probably) only by Settings app.
- super.onActivityCreated() is not called at the beggining of
  WifiSettings#onActivityCreated(), the parent method assumes
  the child should have prepared PreferenceScreen, while
  WifiSettings cannot do until the parent Activity is ready.
- Call SetHasOptionMenu() should be called AFTER the parent
  Activity is ready. It is not documented, so it would be better
  to file another bug.
- Add exception to proguard...

Change-Id: Iebd27f0cb0abdbee9b4b1cc9b00f4bf127f7815d

res/layout/settings_top_level.xml

@@ -24,4 +24,44 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent"
             />
+
+    <!-- button_bar -->
+    <RelativeLayout android:id="@+id/button_bar"
+        android:layout_height="wrap_content"
+        android:layout_width="match_parent"
+        android:layout_weight="0"
+        android:background="@android:drawable/bottom_bar"
+        android:visibility="gone">
+
+        <Button android:id="@+id/back_button"
+            android:layout_width="150dip"
+            android:layout_height="wrap_content"
+            android:layout_margin="5dip"
+            android:layout_alignParentLeft="true"
+            android:drawablePadding="3dip"
+        />
+        <LinearLayout
+            android:orientation="horizontal"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:layout_alignParentRight="true">
+
+            <Button android:id="@+id/skip_button"
+                android:layout_width="150dip"
+                android:layout_height="wrap_content"
+                android:layout_margin="5dip"
+                android:text="@string/skip_button_label"
+                android:visibility="gone"
+            />
+
+            <Button android:id="@+id/next_button"
+                android:layout_width="150dip"
+                android:layout_height="wrap_content"
+                android:layout_margin="5dip"
+                android:drawableRight="@drawable/ic_btn_next"
+                android:drawablePadding="3dip"
+                android:text="@string/next_button_label"
+            />
+        </LinearLayout>
+    </RelativeLayout>
 </LinearLayout>