MSIM: Fix phone app crash

this patch is adoption from CAF 1974e8b40a73101121038a2a7cbfae4974362a85 
by Chaitanya Saggurthi <csaggurt@codeaurora.org>
MSIM: Fix phone app crash

mButtonOperatorSelectionExpand should not be null when
 preference is removed, because when there is a RAT change
 need mButtonOperatorSelectionExpand preference again to
 enable or disable.

Change-Id: I2916e0f31bcf6a7bafafd5f7c3bcee8b29339ed3
CRs-Fixed: 668741