Remove MobileNetworkSettingsTest#onAttach

The MobilieNetworkActivity has MobileNetworkSettings fragment. The
MobileNetworkActivityTest do testcases for activity stability.
The activity's launch test successful is equals the fragment doesn't
crash, so the activity's testcases cover
MobileNetworkSettingsTest#onAttach.

Bug: 147464840
Test: build pass
Change-Id: Idbabb846c40fd117f22a39b86fd6628b7d8c5b1b