gril: Fix 0 signal strength in MTK modems

MTK modems were returning wrongly 0 for the signal strength when that
figure was actually unknown.
unit: Add test for 0 signal strength case
rilmodem: Consider tech when parsing strength

Consider radio technology when parsing signal strength.
Merge pull request #109 from alfonsosanchezbeato/fix-0-strength

Fix no signal strength bars for MTK modem