Fingerprint: Cancel timeout if user leaves enrollment

Also added safety check on null activity just in case.

Repro Steps:
1. Settings > Lockscreen > Manage fingerprints
2. Click "add new fingerprint"
3. Click "found it!"
4. Push back button on device.

After a few seconds of no input, the NPE would occur.

Change-Id: Ida0392262f9e95eea7a9f8df08737e419483a965
Issue-Id: SAMBAR-201