Fix potential exception while pressing volume up and volume down

When enabled a11y service, it would intercept key events and forwards
them to accessibility manager service.
This CL added the synchronized mechanism in KeyCombinationManager to
prevent it access the conflict state because
'interceptKeyBeforeDispatching' and 'interceptKeyBeforeQueueinging' are
called from different threads.

Bug: 209939290
Test: A11y CTS and unit tests.
Change-Id: I7adb70d21128340c5519a6c72cb4d732c0bbba43