Add new public API to allow call screening

This CL adds a new service that the default dialer can implement to
screen incoming calls.

If the service is implemented then Telecom uses it to decide if a call
should be shown to the user or rejected.

Note, I initially tried to simply extend InCallService instead of
creating a whole new service. The problem with this approach is that:
  - this breaks some in call services which show UI as soon as they are
    bound to
  - the in call service lifecycle is tightly coupled to adding and
    removing calls. Call screening happens before a call is added which
    makes it a bad fit to in call service
  - call screening is a function of only the default dialer's in call
    service but the existing code also binds to "controller" call
    services

For these reasons it seemed simpler to just create a new optional
service.

BUG: 22857261
Change-Id: I53f2ef93360e0af19b6ce45af21026be6cf3a7f3