Fix the memory leak caused by the accessibility menu in SystemUI

When you turn on the "Accessibility Menu shortcut" switch in settings,
it starts a service and creates a new A11yMenuOverlayLayout.

However, the layout is not set to null at the right time, causing the
A11yMenuOverlayLayout to be repeatedly created and leading to a memory
leak.

Change-Id: I2368a1bbbbe0159755859651ecf53831ac70db37
Signed-off-by: Jia Jia <jia.jia@zte.com.cn>