IMS-VT: Low Battery Handling for MO and MT Video calls

When UE is under low battery:
1. User decides to place a Video call then,
   Interrupt the MO Video call and seek
   user confirmation to convert the video call to voice call
   or continue a video call
2. User receives a Video call and when user decides to answer
   the call as Video then, seek user confirmation to convert the
   video call to voice call or continue as video call
3. Disconnect the call if user presses back key with
   low battery dialog showing
4. Consume back key press for MT/Active Video call thereby not giving
   the option to dimiss low battery dialog

Change-Id: I0da978c3988d77dd0b6ed2eb9fef8d8fd2f7f43f
CRs-Fixed: 1012524