Fix ConnectivityManager memory leak

ConnectivityManager have a self reference static instance. This
causes a leak if context is refernce from a static variable. Using
the applicationContext, which will never be freed for the life of the
application, for the sInstance static instance.

Bug: 202978965
Test: atest ConnectivityManagerTest
Change-Id: I87206e1bfbb1f877b5a10f5fdbc25e2f9f11bef4

tests/unit/java/android/net/ConnectivityManagerTest.java

@@ -41,6 +41,7 @@ import static com.android.testutils.MiscAsserts.assertThrows;
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.mockito.ArgumentMatchers.anyBoolean;
@@ -506,8 +507,7 @@ public class ConnectivityManagerTest {
             Thread.sleep(waitIntervalMs);
         }
 
-        // TODO: fix memory leak then assertNull here.
-        assertNotNull("Couldn't find the Context leak in ConnectivityManager after " + attempts
-                + " attempts", ref.get());
+        assertNull("ConnectivityManager weak reference still not null after " + attempts
+                    + " attempts", ref.get());
     }
 }