Add config value to switch between Tether Settings UIs.

Adding this config value to make it easy to switch to developing
Tethering settings UI. Also, using this config value to make sure
that preferences from new AllInOneTetherSettings screen won't show up in
the search results.

Bug: 147915366
Test: make RunSettingsRoboTests ROBOTEST_FILTER=CodeInspectionTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=TetherSettingsTest
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=TetehrPreferenceControllerTest
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiTetherSettingsTest

Change-Id: I8c88816a8f2b40fc219cae8d0d442454c5270b2b