Wi-Fi network details page fixes.

- Fix NPE in WifiDetailPreferenceController#setIpText
- Add MAC address preference in the details section
- Set subnet mask and gateway preferences
- Do not show IP information preferences if no information is available
- Fix string capitalization errors
- Only show IPv4 DNS servers under "Network details" section

Bug: 36483230
Bug: 37096448
Bug: 36482499
Bug: 37165860
Test: m RunSettingsRoboTests
Change-Id: I0e3f0ccfc4a8d802b51ed6b3be81c75e384dd06f