7/n: BiometricService is the source of truth for sensor strength

Individual sensor services only need to keep track of their
ID. When doing things that require knowledge of current strength,
query BiometricService.

Bug: 149067920
Fixes: 151953729

Test: atest com.android.server.biometrics
Test: BiometricPromptDemo

Change-Id: Id2b7cd4e02c1a4b4707950285923f8166b8f46e1