Add a flag for moving UdfpsEnroll* from SystemUI to settings.

- Adds SETTINGS_SHOW_UDFPS_ENROLL_IN_SETTINGS flag to guard the change.
- Remove the black fp drawable in udfps_fpm_other_view.xml
- Uses UdfpsFpmOtherViewController when the flag is enabled, so that SystemUI doesn't show anything for settings enrollment.

Turn this flag on via adb:
adb shell setprop sys.fflag.override.settings_show_udfps_enroll_in_settings true

Test: manually tested on device
Bug: 260617060
Change-Id: I1842c4351041fdabc16bde9d5d5a708efb258cb2