Add an error dialog in FingerprintEnrollFindSensor

When a device stands for a while during the fingerprint enrollment, the
fingerprint icon will disappear without any warning message. This
change is trying to show a dialog to inform users.

Fixes: 180633056
Test: visual verified
1) Settings -> Security -> Fingerprint
2) Navigate to "Touch & hold the fingerprint icon" page
3) Let device stand for a while and see if there's a dialog shown

Change-Id: I69c8fb73f2a7406e4a379396e2624fb523e162a8