[FIXIT] Add JUnit coverage to DeviceConditions

* Mostly adds tests covering 100% of DeviceConditions
* Makes NetworkChangeNotifier expose resetting of its instance to
  tests outside of its direct package (to enable unit testing).

Bug: 1066565
Change-Id: I20012d10e3b55a903fb15b65a39d5ef50359e5ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128717Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755383}

net/android/java/src/org/chromium/net/NetworkChangeNotifier.java

@@ -75,7 +75,8 @@ public class NetworkChangeNotifier {
         return sInstance != null;
     }
 
-    static void resetInstanceForTests(NetworkChangeNotifier notifier) {
+    @VisibleForTesting
+    public static void resetInstanceForTests(NetworkChangeNotifier notifier) {
         sInstance = notifier;
     }