Fix vr_hwc crash when re-entering vr

An enter, exit, enter vr sequence would result in a crash in vr_hwc
because we weren't clearing the event callback pointer in
unregisterEventCallback() during the exit vr step. When we would enter
the second time we'd try to call onRefresh() on the stale pointer,
resulting in a crash.

Bug: 77498352
Test: Enter, exit, enter vr, confirmed no crash.
Change-Id: If93aaf9935844565cb99bbf7349573a8ddfff4ca