Stop AuthContainerView from crashing with 2 sensors

Fixes an issue with BiometricPrompt crashing on devices with two sensors
(e.g. face and fingerprint) by using whichever prompt UI corresponds to
the first sensor in the list. Later commits will improve on this UI to
indicate when the actual authentication modality changes.

Test: Manually tested; verified the prompt no longer crashes.

Fixes: 173540555
Change-Id: I2add151fe336e53929a2f8167213b773d1fd48d4