Fix bug where fingerprint events can be delivered to the wrong client

- Make FingerprintService more closely track the expected state of fingerprintd.
- Don't switch to a new operation until fingerprintd completes previous operation.
- Refactor clients into separate classes and add tracking logic.
- Add missing enumerate()/cancelEnumeration() methods to IFingerprintDaemon
- Make late-binding decision of "foregroundness" of activity so that it's
decided in the order the events are actually handled.
- Add more logging so we can determine FingerprintService state when errors occur.
- Cache a copy of authenticator_id from the last time it was set so we don't
interrupt the driver during actual authentication.
- Don't allow clients to access authenticator_id unless they're current.

Fixes: 27902478, 26273819
Change-Id: Ic1f9e30bd89bcdbb8fe7f69e0545e68339317721