a11y: Pause auto click with pause button

Pause autoclick when the pause button is clicked by canceling
any pending clicks and preventing scheduling of new clicks.

Demo: http://b/388872274#comment4

Bug: b/388872274
Test: AutoclickTypePanelTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I1cabc63a2656ea4e962a5565b84e5db70eb115ae