Center mode options according to spec.

* spec: https://drive.google.com/a/google.com/#folders/0B0GnUi_SFKJtdHp4YnBnLTZxcjQ
* for modes like video with less than the total possible
  mode options, center the options.

Change-Id: I79dce62ced916d2ed20d04e805abe2bfdd60c779
diff --git a/src/com/android/camera/ButtonManager.java b/src/com/android/camera/ButtonManager.java
index 8e8ba7d..526ffd4 100644
--- a/src/com/android/camera/ButtonManager.java
+++ b/src/com/android/camera/ButtonManager.java
@@ -406,7 +406,7 @@
             button = (View) getImageButtonOrError(buttonId);
         }
         if (button.getVisibility() == View.VISIBLE) {
-            button.setVisibility(View.INVISIBLE);
+            button.setVisibility(View.GONE);
             if (mListener != null) {
                 mListener.onButtonVisibilityChanged(this, buttonId);
             }