Add phenotype namespace and flag for biometrics

This allows the ability to enforce biometric modality strength (downgrade only) on certain devices.

Bug: 141025588

Test: atest com.android.server.biometrics
Test: make system-api-stubs-docs-update-current-api
Test: make test-api-stubs-docs-update-current-api
Test: manual testing using "adb shell device_config put NAMESPACE KEY VALUE"

Change-Id: Ib167a038d717216ec0a54d6f4f6f5f88a70e1a6e