Use consistent logic for data icon overlays.

Ensure that we always show the data icon in both Quick Settings and
the status bar, or in neither, but never one or the other.

The particular inconsistency this is intended to fix is that in some
circumstances, Wi-Fi may be connected, but the cell radio may be still
be used by certain apps, due to new multinetworking APIs in L. In this
case, we should always show the data icon; currently appears in the
status bar, but not Quick Settings, which was unconditionally dropping
the icon if any Wi-Fi connection was up.

Bug: 19112322
Change-Id: I9942f6b24081e061a72804ef47ad4fe719f32ec2