Create a fragment class for the new "Hotspot & tethering" screen

This fragment class is created to display Wifi tether settings, Usb
tether settings and bluetooth tether settings within the single "Hotspot &
tethering" screen. By creating a new fragemnt, we can safely change the
UI by replace fragment name in xml file.
Right now, it only supports wifi tether settings and mostly copied from
WifiTetherSettings.java.

Bug: 146447611
Test: AllInOneTetherSettingsTest is created and run.
Change-Id: Icdac0d4930451e6956dbf6ef0f4b65bce96fed3f