Unregister hotspot receiver in TetherService.

- when the service exits, also unregister the hotspot receiver.
- move TetherService and HotspotOffReceiver to wifi.tether package.

Change-Id: I0044a52bf80a5530a58b5186ab056de55e83532b
Fixes: 69844871
Test: make RunSettingsRoboTests
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 51a811d..9e4dc39 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -476,7 +476,7 @@
             </intent-filter>
         </activity>
 
-        <service android:name=".TetherService"
+        <service android:name=".wifi.tether.TetherService"
             android:exported="true"
             android:permission="android.permission.TETHER_PRIVILEGED" />