More telephony updates for mainline.

Add new TelecomManager#getDefaultDialerPackage which is multiuse aware;
this is used when showing the voicemail notification.
Remove some @hide methods from Conference; push these inline.
Move ConferenceParticipantConnection into frameworks/opt/net/ims since
it is just an IMS implementation detail.

Bug: 141576016
Test: Manual smoke test.
Test: Run unit tests.
Test: Run CTS tests.
Change-Id: I39b6955cb14cc1ca68b05c620c3d09a2cdfe30c9
Merged-In: I39b6955cb14cc1ca68b05c620c3d09a2cdfe30c9