Improve delete fingerprint UI.

- Make fingerprint preference use two_target preference
- And add a delete button as secondary target
- Remove delete fingerint code from RenameDeleteDialog, and move to new
  DeleteFingerprintDialog
- Some code cleanup
- Tests

Change-Id: I49d9c7d1d03497f07c7e20a805f23e6da934f587
Fix: 37783186
Test: make RunSettingsRoboTests