[fenix] Fixes https://github.com/mozilla-mobile/fenix/issues/1270 - Prevent tapping on hidden buttons

app/src/main/res/xml/home_scene.xml

@@ -104,6 +104,7 @@
         <Constraint
             android:id="@+id/menuButton"
             android:alpha="0"
+            android:visibility="gone"
             android:layout_marginTop="16dp"
             android:translationY="-10dp"
             android:layout_height="@dimen/glyph_button_height"
@@ -114,6 +115,7 @@
         <Constraint
             android:id="@+id/privateBrowsingButton"
             android:alpha="0"
+            android:visibility="gone"
             android:translationY="-10dp"
             android:layout_height="@dimen/glyph_button_height"
             android:layout_width="@dimen/glyph_button_height"