Update preference text for Network & internet screen.

- Add footer info to Hotspot & tethering
- Add summary text for Data usage -> Network restrictions
- Update summary for Hotspot & tethering -> Portable Wi-Fi hotspot
- Remove quotes from Wi-Fi summary

Bug: 36296257
Test: make RunSettingsRoboTests

Change-Id: Ibab61b089521a3339e660e9df0db0cd1b316712b