Use feature flag to show/hide AllInOneTetherSettings

This partially reverts commit 0ccc849d
which added a config value. Instead, we will use feature flag to switch
between the fragments.

This CL also adds a postfix to keys in all_tether_prefs to
de-duplicate with keys in tether_prefs and wifi_tether_settings.

Bug: 148182953
Change-Id: I92832c786473990065a965409072e4117a7e75a8
Fix: 148618984
Test: make RunSettingsRoboTests
Test: make RunSettingsRoboTests ROBOTEST_FILTER=CodeInspectionTest