Update biometric constant docs from tier to class

Same as ag/11711578, but updates the names of the biometric security
constants in documentation from "Tier 1/2/3" to "Class 1/2/3". This
matches the latest language in the Android 11 CDD.

Test: m -j
Bug: 157266312
Change-Id: Ic5c5674232737f468aa750e60c60dd6a3637a6f8