Fix signal strength display in 'Choose network' list

For GSM networks, the "Choose network" page allows you to manually pick
from among different cell networks, and shows signal strength and
network type (LTE, 3g, etc.) for available ones. The signal strength
wedges were transparent and not visible to the user - this CL fixes
that.

Bug: 132895135
Test: visual (Insert SIM with GSM service, then go to Settings ->
Network & internet -> Mobile network -> Advanced -> Automatically select
network)

Change-Id: Ibbbc60b590b09e4b4f5e31b49fe578769dcdbc4d