Fix back navigation in biometric enrollment.

Using the back buttons can cause a crash in at least two cases. Skipping
face enrollment and then starting/stopping any enrollment can lead to
an invalid token and failed HAT request. Backing out of the activity and
restarting it can also lead to using a stale token that fails.

Fix: 179336333
Test: manual on device
Change-Id: I0c1133e4c3d9c97997043ddc9374aa3cfc4f1c97