Do not remove enumerate client immediately when user switches

The client should not be removed until ERROR_CANCEL is removed in the
system service, since that's the HIDL contract. Pending clients will start
after, automatically with the current logic.

Test: With fingerprint enrolled on multiple users, switch rapidly
      system service does not crash

Fixes: 128346522
Change-Id: I6907d8f76373c012ddffd2b98a81d70e79ac72ab