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
Merge remote-tracking branch 'github/cm-12.0' into HEAD