Dialer app changes for IMS

IMS Conference URI UI changes
1. Add a new button for initiating conference
URI call in the Dial screen.
2. Send a intent for the conference call with a
extra to indicate the call type as conference.

IMS: Allow dial Conference call without URI List

Allow place conference call without URI List and
don't show error toast in this case

IMS: Clean up Conference URI system property

Remove system property "persist.dbg.ims_volte_enable".
Add platform configuration flag and user's enhanced 4G
LTE setting to decide IMS capability.

IMS: Add participant support

Insert "add_participant" extra
to dial intent

IMS: Fix dialer app crash issue due to NPE

FragmentActivity can be null if it's not yet
attach. So it can be null in such case.
Add null check to avoid NullPointerException.

Change-Id: Ib35c394dd10dea7526601824f11e5ebd9db459ce