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.

Change-Id: I1ee4dfb8e73ae7bf72a3daafef6957736a560606
CRs-Fixed: 727689
Contacts/Dialer/InCallUI: Dark theme, (4/7):

Part 2/7: frameworks/base
Part 2/7: Contacts
Part 3/7: ContactsCommon
Part 4/7: Dialer
Part 5/7: InCallUI
Part 6/7: PhoneCommon
Part 7/7: services/Telephony