Add authenticatorId changes to Fingerprint aidl

Also adds a top-level README for the biometric package

Bug: 159667191
Bug: 168541000

Test: make -j56 android.hardware.biometrics.fingerprint-update-api
Test: make -j56 VtsHalBiometricsFingerprintTargetTest

Change-Id: I3dba8e7ee16700865662c1a00b75576d2513f930