Reword brightness cap API to be aligned with display service variant

Brightness clamping API in display service follows a range from 0 to 1
and this CL brings the notification manager aspect of it in line with
the one for display service for consistency

Fixes: 427541717
Test: presubmit
Flag: EXEMPT trivial
Change-Id: Ibbd616f4e51924e6fdf04e268723390312ae316c