IMS-VT: Multitasking feature

-Support for video multitasking

Change-Id: Ic031ae1681a4bb8299a13a5e8187aaff4f27374f
Propagate the call substate changed message to the UI

Change-Id: I2e2b0dd5722ab9d340d10044ad52ba8fee2effe2
CRs-Fixed: 749824