Fix Javadoc and null analysis related warnings

This CL also adds @SuppressWarning("unused" to java-overridable package.

Bug: 18003991
Change-Id: If70527e30654384705d7a814f5efd181d9f539e1
diff --git a/java/res/values/keyboard-themes.xml b/java/res/values/keyboard-themes.xml
index b0bae96..26b2582 100644
--- a/java/res/values/keyboard-themes.xml
+++ b/java/res/values/keyboard-themes.xml
@@ -25,7 +25,8 @@
         <item>@string/keyboard_theme_holo_white</item>
         <item>@string/keyboard_theme_holo_blue</item>
     </string-array>
-    <!-- An element must be a keyboard theme id of {@link KeyboardTheme#THEME_ID_*}. -->
+    <!-- An element must be a keyboard theme id of
+         {@link com.android.inputmethod.keyboard.KeyboardTheme#THEME_ID_ICS} etc. -->
     <integer-array name="keyboard_theme_ids" translatable="false">
         <item>3</item>
         <item>4</item>