Track userId independently from callback for fingerprint.remove().

If we get an extremely delayed response from the fingerprint
hardware or the system crashes, it was possible to get into
a state where we have no remove ClientMonitor and FingerprintService
would crash.

The fix is to track the userId for removal independently
from the callback.

Fixes bug 28028434

Change-Id: I2b3e3eb332ea5512b86d3f7be853b720e41318a4