Switch from FingerprintService to FingerprintManager.

FingerprintService is a lower layer of abstraction which should
ideally be accessed only via FingerprintManager from
AndroidKeyStore.

The main issue with the switch is that it requires a reference to a
Context. This is now obtained using ActivityThread's hidden API.

Change-Id: If921e169838ee2cc5c7690b8c8d8ea95c33248aa