Synchronize on mImsPhone to avoid NPE

During a disconnect, the binder thread from the IMS stack may set
mImsCall to null simulataneously with the main thread calling methods on
mImsCall after a null check. Adding synchronization to prevent an NPE.

Change-Id: Id7874e802d56ab7d3912c779dedd1cccdb923db6
Fixes: 37945103