EthernetTetheringTest: join handler thread after calling quitSafely

Somewhat surprisingly, HandlerThread#quitSafely is *not* a synchronous
method, so the tearDown() method still needs to wait for the thread to
join.

Test: TH
Change-Id: I4297ba057b3d4e67c7b462b6995da637f2479d42
diff --git a/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java b/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
index f2cf1d0..69eb58f 100644
--- a/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
+++ b/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
@@ -253,6 +253,7 @@
             if (mRunTests) cleanUp();
         } finally {
             mHandlerThread.quitSafely();
+            mHandlerThread.join();
             mUiAutomation.dropShellPermissionIdentity();
         }
     }