Add index for Hotspot settings page into search

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