SystemUI: Fix data activity overlay not being removed when data is disabled

If mobile data connection is disabled while there is active data
activity, the data activity overlay will not be removed and will
be stuck there. This patch fixes that.

(Previously merged 5c39b1ce but it was
accidentally removed in 874c5d8c due to
merge error.)

Change-Id: I0eda652a7533fed8db58d7dca2ba60b0b28a4b81