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

res/xml/tether_prefs.xml

@@ -17,6 +17,7 @@
 <PreferenceScreen
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:settings="http://schemas.android.com/apk/res-auto"
+    android:key="tether_prefs_screen"
     android:title="@string/tether_settings_title_all">
 
     <com.android.settings.widget.FixedLineSummaryPreference
@@ -29,12 +30,14 @@
     <SwitchPreference
         android:key="usb_tether_settings"
         android:title="@string/usb_tethering_button_text"
-        android:summary="@string/usb_tethering_subtext" />
+        android:summary="@string/usb_tethering_subtext"
+        settings:keywords="@string/keywords_hotspot_tethering" />
 
     <SwitchPreference
         android:key="enable_bluetooth_tethering"
         android:title="@string/bluetooth_tether_checkbox_text"
-        android:summary="@string/bluetooth_tethering_subtext" />
+        android:summary="@string/bluetooth_tethering_subtext"
+        settings:keywords="@string/keywords_hotspot_tethering" />
 
     <Preference
         android:key="disabled_on_data_saver"

res/xml/wifi_tether_settings.xml

@@ -18,6 +18,7 @@
 <PreferenceScreen
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:settings="http://schemas.android.com/apk/res-auto"
+    android:key="wifi_tether_settings_screen"
     android:title="@string/wifi_hotspot_checkbox_text"
     settings:initialExpandedChildrenCount="2">
 
@@ -43,4 +44,4 @@
         android:dialogTitle="@string/wifi_hotspot_ap_band_title"
         android:negativeButtonText="@string/cancel"
         android:positiveButtonText="@string/apply"/>
-</PreferenceScreen>
+</PreferenceScreen>