Fix keyboard focusability on title bar.

Even when the title bar is visible, those components (including
maximize button and close button) shouldn't be keyboard accessible.
Usually those functions are accessed through key combinations
such like maximize-button or global shortcut keys. Those buttons
should exist for mouse/touch accessibility.

Bug: 23590300
Change-Id: Id61d4d5c889f1a657befdc12494993a2a19685b2