Fix the crash due to multi-register callbacks

For multi-display device, like pixel_jumbojack, when this device is
folded, the secondary display is removed. The A11y framework stops
tracking windows on the removed display by un-registering its
callback.

When the A11y framework un-registers a callback, it checks if the
display is embedded. If it is, the observer is not removed since
the same observer will be observing the parent display.

When the display disappears entirely, however, its context
disappears and there's no way to know if it was embedded or not.
The observer was therefore not removed. When the device is unfolded
and the display reappears, the attempt to register an observer
causes an exception because the previous one is still there.

Due to there's no multi-display device for android S now, so we
adopt simple solution as below to fix it:
1. Throws the exception for debug builds only.
2. Adds this exception into the error log.
3. Recovers by unregistering the current observer and proceeding to
register the new one

The fully solution will phase in the master.

Bug: 182963008
Test: a11y CTS & unit tests
Change-Id: Iff9776ac6e054c0198c0d8739d9446a785716612