Added index provider into TetherSettings and WifiTetherSettings pages.
Added robotests to check the keys returned by getNonIndexableKeys.
Move repeated code into utility class.
Bug: 74777873
Test: make -j RunSettingsRoboTests
Test: atest UniquePreferenceTest
Change-Id: I6cb09d735afa4867d25a79de733ce8c59634e49b