SystemUI: Fix "wifi icon overlaps mobile signal icon" issue for single sim

The mobile signal icon is not triangle in carrier style, so it's necessary
to insert a spacer between wifi icon and sim mobile signal icon.

Use the spacer if the signal style is not default.

CRs-Fixed: 638161

Change-Id: I4b52c6047e34f1904a2f97a98eff38324ea59928