Wait for fingerprints to be removed (Partial fix).

This is a partial fix for b/27903189.
When we remove the lock screen and remove all fingerprints, wait for
them to all be removed before finishing the activity. This will let
the security screen accurately show how many fingerprints are available.

bug:27903189
Change-Id: I30908dbefb7a858f6d99e532841ed4ff894bfe62