SystemUI: Fix the wrong speech in TalkBack application.

A missing '@' while accessing the string value
in arrays.xml causes the UE to speak resource name
of signal strength instead of string value.
Add the missing '@' in arrays.xml of SystemUI to
fix the wrong speech in TalkBack application.

Change-Id: Ic2a08a106ecc42264254b190f21306915cb40192
CRs-Fixed: 778164.