Add tethering event callback API

Provide OnTetheringEventCallback for system app to know
tethering's upstream.

Bug: 125583822
Test: -build, flash, boot
      -atest FrameworksNetTests

Change-Id: I7ca81b27c9b805cc01884509f5b20d9d0a24cd36
diff --git a/Android.bp b/Android.bp
index fb6ff0f..5e03982 100644
--- a/Android.bp
+++ b/Android.bp
@@ -225,6 +225,7 @@
         "core/java/android/net/INetworkScoreService.aidl",
         "core/java/android/net/INetworkStatsService.aidl",
         "core/java/android/net/INetworkStatsSession.aidl",
+        "core/java/android/net/ITetheringEventCallback.aidl",
         "core/java/android/net/ITetheringStatsProvider.aidl",
         "core/java/android/net/nsd/INsdManager.aidl",
         "core/java/android/nfc/IAppCallback.aidl",