Fix in the scrollable Glif header design, the sub-title can not

be read out automatically in the Udfps enrolling page

1. To set focus on description view when turned on A11y &
description view invisible.
2. Get back the hiding description snippet and wrap it by
aconfig flag condition.

Flag: com.android.settings.flags.enroll_layout_truncate_improvement

Bug: 379090673

Test: atest UdfpsEnrollEnrollingView
Test: Build Forrest ROM then check the UI in different scenarios
Change-Id: Ic76c15ccb82998781102492ef9894190aa824218