Added support to authentication.

AutoFillService can now require user authentication, both at
FillResponse and Dataset levels;
- FillResponse authentication is typically used when the user data
  need to be unlocked before the first use.
- Dataset authentication is typically used to unlock sensitive data
  such as credit card info.

The authentication can be handled by the service itself (for example,
when it uses the credit card CVV to unlock it) or by the Android
system (when the service asks for fingerprint authentication).

Bug: 31001899

Test: manual verification
Test: CtsAutoFillServiceTestCases passes

Change-Id: If62f42f697ab5ef0d14d991ff1077d1c38808e61