Fix null drawable for Mobile signal data edit QS tile

Currently it's a funky situation with CellTileView. There should
ultimate be a better way to handle displaying the SignalDrawable (per
the TODO comments), but this is a sensible fix to show the mobile data
icon in the edit QS screen for now.

Test: visual
Bug: 62525439
Change-Id: I95f0fb86862aa6a7b60f304053a73127907943ee