LatinIME: Fix invisible icon

* Settings -> System -> Languages & Input ->
  On-screen keyboard -> Android-Keyboard (AOSP)
  -> Appearance & Layouts -> Customized
* The add-icon in the actionbar is close to
  invisible on light theme, since the color is
  hardcoded to white
* Import the same add icon as vector drawable
  instead and set the fillColor to
  colorControlNormal - that way it's looking
  properly on both, dark AND light theme

Change-Id: I8fa23f0efaa6c7ab6982eeab64aad8bf86628153