Connectivity Tests: 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: I0433e2100adf9299a67af5c259a518cf44f4be57

tests/common/java/android/net/NetworkProviderTest.kt

@@ -79,6 +79,7 @@ class NetworkProviderTest {
     @After
     fun tearDown() {
         mHandlerThread.quitSafely()
+        mHandlerThread.join()
         instrumentation.getUiAutomation().dropShellPermissionIdentity()
     }