DO NOT MERGE Adding unit tests for ExternalCallList.

- Also adding TestTelecomCall class, which is a reflection wrapper that
can create instances of android.telecom.Call.  These calls are particularly
problematic as they are final, so Mockito cannot mock them.

Bug: 27458894
Change-Id: Ibd3786dc27eb4280eb32b8fb0baa18d42738a98c
(cherry picked from commit 5df8de7b)