Unregister IA11yManagerClient in ToastPresenter

AccessibilityManager is used to send an a11y event when the Toast is
shown, so unregister the client after it's shown

Test: Track clients in A11yManagerService before and after
multiple toasts are shown with adb shell dumpsys accessibility,
atest ToastUITest
Bug: 189010828

Change-Id: Ie4759d2f67ab9e3149d44169b812d8ec2316c266