Fix NPE in updateAddPreference

Update mAddFingerprintPreference to avoid NPE

Fixes: 286495189
Test: Rotate screen during first fingerprint enrollment, complete enrollment, and observe no crash
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f58714496ab5454ea10c6d638c094bb1d21306d8)
Merged-In: Id7edde492168b467360c6c99b326721cd883bba8
Change-Id: Id7edde492168b467360c6c99b326721cd883bba8
1 file changed