Add support for different icon in legacy menu

Action icons that work well in a dark action bar usually don't work well
in the legacy menu of Android 2.x. This patch allows to specify a
second icon that will be used in the legacy menu.

Example:
<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto" >
<item
  android:id="@+id/add"
  android:icon="@drawable/ic_action_add"
  app:legacyIcon="@drawable/ic_menu_add"
  android:title="@string/add" />
</menu>