Remote Conference Callbacks tests.

Adding CTS tests to test all the RemoteConference callbacks.
Misc changes:
1. Adding super() invoaction on all callbacks to ensure that they're
detected by the CTS coverage tool.
2. Changing the conference object used in the Conference test to be vanilla
Conference object so that it's usage can be detected by CTS tool.

BUG: 20303674
Change-Id: Ie3409aeaaaf319add9e84d7cdfe641bdaa68dc8e
3 files changed