Change Tethering package name

".apex" is easy to confuse.
Rename as below:
apex: com.android.tethering.apex -> com.android.tethering
apk: com.android.tethering -> com.android.networkstack.tethering

Bug: 146471733
Test: build

Change-Id: I2c7647abb996539a3af6cfe0e0214a5e8927c0d6
diff --git a/Tethering/apex/AndroidManifest.xml b/Tethering/apex/AndroidManifest.xml
index 7769b79..5c35c51 100644
--- a/Tethering/apex/AndroidManifest.xml
+++ b/Tethering/apex/AndroidManifest.xml
@@ -15,7 +15,7 @@
  * limitations under the License.
  -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-  package="com.android.tethering.apex">
+  package="com.android.tethering">
   <!-- APEX does not have classes.dex -->
   <application android:hasCode="false" />
   <!-- b/145383354: Current minSdk is locked to Q for development cycle, lock it to next version