IMS:Re-Initiate Conference Call from Leaf Node Causes Conference Call Crash.

During conference call, if there is an incoming call and user accepts it,
the ims FW triggers two accept commands since ims FW triggers the onCallHeld
multiple times for conference call.
The multiple accept commands causes the ims ril to respond failure. As the result,
the call drops and also crash occurs.
The fix is to block the multiple accepts in ims FW layer.

Bug: 17606426
Change-Id: I89904d4c64dad0d19398f74408ddc177f59c40ce