Verify fingerprint exists before using it

The process will crash if we try to obtain a FingerprintManager
on a device that doesn't implement one. Verify that the feature
exists before trying.

Bug: 34923693
Test: Ran A11y unit tests and a11y fingerprint cts.
Change-Id: I48ad1c2c9c0c5739d8b1826c3c53bb817078e6cf