Fix for potential deadlock in EventThread::onNewVsyncSchedule

I haven't seen such a deadlock, but looking over
I86e66df59c64e81c4aa721a25250697f61237488 and the potential deadlock
that inspired it, I think there could be a similar problem in
EventThread. As I understand it, this is partially because of the
fact that EventThread does not unregister the old callback. This is
addressed in I3c1eccb36914f29560600d48bb08b1b8f2fe7c96.

Fix the deadlock with a similar approach as
I86e66df59c64e81c4aa721a25250697f61237488: hold on to the old
registration until after releasing the mutex.

Bug: 279209321
Test: infeasible
Change-Id: If490f88115aa298d31aa9ad392a1f9f9dc987549