Add toggle for Wi-Fi hotspot

- Add Wi-Fi hotspot toggle with vertical divider

- Disable preference and toggle when Data Saver is enabled

Bug: 245569117
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=TetherSettingsTest
make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherPreferenceControllerTest
atest -c TetheringManagerModelTest

Change-Id: Ic2baf7d3d0a7bf9527da38d24ecc511b7c91265a