Use '!' instead of 'x' in signal drawable

Update the not-connected iconography to use an exclamation mark instead
of the 'x'.

Test: manual; adb shell am broadcast -a com.android.systemui.demo -e command network -e mobile show -e level -2 -e datatype lte
Fixes: 175335798
Change-Id: Iff4b7d9e7fe113c50b92a45cace3a742f7af42f6