[Testing] Remove Robolectric import and use AndroidJUnit4 instead in MobileDataEnabledListenerTest

1. Use AndroidJunit4 instead of RobolectricTestRunner
2. Use ApplicationProvider instead of RuntimeEnvironment to get context

Bug: 162048313
Test: atest -c MobileDataEnabledListenerTest
Change-Id: I81b3d6ac075666a45649c8d811832a40bdb1b41b
diff --git a/tests/unit/Android.bp b/tests/unit/Android.bp
index 583b46e..7556cbe 100644
--- a/tests/unit/Android.bp
+++ b/tests/unit/Android.bp
@@ -17,6 +17,7 @@
         "androidx.test.espresso.core",
         "androidx.test.espresso.contrib-nodeps",
         "androidx.test.espresso.intents-nodeps",
+        "androidx.test.ext.junit",
         "mockito-target-minus-junit4",
         "platform-test-annotations",
         "truth-prebuilt",