Update biometric test paths

1) Test HALs should always respond to enumerate with no enrollments.
   CTS tests use this path to consciously remove (framework)
   enrollments when cleaning up.
2) Removes unnecessary TestSession classes from AIDL providers. These
   are redundant with the existing sessions returned from the TestHals
   themselves. We just need the framework to retrieve a new HAL session
   (from either the real HAL or the test HAL) whenever changing between
   test and non-test modes

Test: atest CtsBiometricsTestCases
Test: atest com.android.server.biometrics
Fixes: 179949545
Fixes: 179856730

Change-Id: I1bbf0b77c69a36c9065193d10be32e942af8db70