Set pending client to null after it's been started

Pending client can now be started in two places. Originally it was only
started when onError was received (previous client canceled). We recently
discovered some enumerate bugs likely due to system boot timing changes,
where Keyguard authenticate() can occur while enumerate() is still going.

As a result, we queue clients while enumerate is happening and start them
when enumerate finishes. We need to remember to set the pending client to
null.

Fixes: 130208531

Test: Fingerprint works during cold boot
Change-Id: I5fe1c7a9fc96875f3d87048b12e601755d7f4963