Add classes/tests for parallel call-filtering lookup

Adds the IncomingCallFilter class, which serves as a controller for the
call-filtering operations. Also adds DirectToVoicemailCallFilter, which
is an implementation of the CallFilter interface defined in
IncomingCallFilter.

No changes are made to existing functionality. Those will be coming in
a future CL.

Bug: 26883888
Change-Id: Idf6662a9e71de60a87d1fc6b6b148fe9a2b12a8e