1/n: Move modality-specific files into new subdirectory

It's easier to read/maintain if we separate sensor-specific code
from BiometricPrompt/BiometricManager code

Bug: 157790417

Test: Enroll/auth fingerprint/face on respective devices
Test: atest com.android.server.biometrics

Change-Id: I472c206743482bbea75498051c6f81e1fbd351ad