Add face enroll accessibility dialog

When accessibility services such as talkback are enabled, and the
user tries to start the non-accessibility enrollment flow, present
a confirmation dialog.

Fixes: 152633740
Test: Enable talkback, start enrollment
      1) Accessibility flow --> no dialog, as expected
      2) Non-accessibility flow --> new dialog shown
Test: No talkback, start enrollment. No dialog shown in either case
Change-Id: I0cd07a9d0012f6c9bea36e74365a6707755d3ab7