Add TetheringTests to postsubmit

The tethering unit tests were in FrameworksNetTests, but were moved out
so were no longer run in presubmit. Now put them to postsubmit first.
Should be moved to presubmit after a while.

Bug: 148998835
Test: atest --test-mapping frameworks/base/packages/Tethering:postsubmit

Change-Id: I9ca36cba7bfde8409a06dbeb7166337c55b39691

packages/Tethering/TEST_MAPPING

@@ -0,0 +1,7 @@
+{
+  "postsubmit": [
+    {
+      "name": "TetheringTests"
+    }
+  ]
+}