Fix ProximityCheck so that re-entrant check calls callback.

This is a followup for I9083d6e87ad199099c117f00dcdc9e7e811435f5.
The test for re-entrant `check` calls, demonstrated that the original CL fixed the ConcurrentModificationException, but also that the check() callback was never invoked. Unregistering before the foreach fixes that.

Test: atest ProximityCheckTest
Flag: EXEMPT bugfix
Bug: 354293532
Change-Id: If4f6d0bd072b0aecf49e8b0e19e358a5a34409a4