[Tether07] Migrate Tethering into module

Now tethering would be run in dedicated service.
TetheringManager is the interface used to communicate with
TetheringService. The new call flow would be: ConnectivityManager
-> ConnectivityService -> TetheringManager -> TetheringService.
Note: the return value of #tether(), #untether() and #setUsbTethering()
APIs would always be no error. Client can use #getLastTetherError()
or #getTetheredIfaces or listen tether state change to check
status of corresponding interface.

Bug: 136040414
Bug: 144742179
Test: -build, flash, boot
      -atest TetheringTests
      -atest FrameworksNetTests

Change-Id: I7e78c0e0a3e70f940a749ba2a39ece7c7ec5b9b3
Merged-In: I7e78c0e0a3e70f940a749ba2a39ece7c7ec5b9b3
diff --git a/services/core/java/com/android/server/net/NetworkStatsService.java b/services/core/java/com/android/server/net/NetworkStatsService.java
index 08c9426..e473c96 100644
--- a/services/core/java/com/android/server/net/NetworkStatsService.java
+++ b/services/core/java/com/android/server/net/NetworkStatsService.java
@@ -138,7 +138,6 @@
 import com.android.internal.util.IndentingPrintWriter;
 import com.android.server.EventLogTags;
 import com.android.server.LocalServices;
-import com.android.server.connectivity.Tethering;
 
 import java.io.File;
 import java.io.FileDescriptor;