Rename “Mobile data usage” to “App data usage” & hide summary

In Data usage screen, rename “Mobile data usage” to “App data usage” &
hide summary.

Also, change the title of the new "App data usage" (old "Mobile data
usage") screen to "App data usage".

Test: manual
Test: make RunSettingsRoboTests
Bug: 70950124
Change-Id: Ide9bcb2a4b171edcd088d68275bacda1eb449afe