Fixes autoadvance on find sensor page for fingerprint.

OnStop gets called after an activity behind it on the stack is resumed
when the user pressed the back button on the navigation bar. Because of
this, we need to disable the cancelEnrollment call inside onBackPressed
instead of onStop so that on the find sensor page, the auto next advance
on pressing the fingerprint sensor will still work.

bug:27151472
Change-Id: I1e440f1f7cd91c0711dd31b8bf65a567d4d39f8b