Add ImsExternalCallTracker and support for multiendpoint.

- Adding new ImsExternalCallTracker which is responsible for responding to
dialog event package information received via IMS framework.
- Added TelephonyTester intents so that I can inject fake dialog event
package data into the ImsExternalCallTracker.

Bug: 27458894
Change-Id: Icc7c4f9e25ad553107fc1cb9d5501ba45c5a1e45