Set background of all face enroll status/footer bars

Moves the logic for setting an explicit background on the status and
footer bars shown during face and fingerprint enrollment from
BiometricEnrollIntroduction to BiometricEnrollBase. This ensures that
the status and footer bars on all biometric enroll screens will have
their backgrounds set, preventing the screen content from showing
through.

Test: Manual

Fixes: 190138494
Change-Id: If5050d07d277f51c784daecf80d5e3cdeae0ba49