make view model instance as private in MainActivity (#1621)

Author: momintahir

app/src/main/kotlin/com/google/samples/apps/nowinandroid/MainActivity.kt

@@ -75,7 +75,7 @@ class MainActivity : ComponentActivity() {
     @Inject
     lateinit var userNewsResourceRepository: UserNewsResourceRepository
 
-    val viewModel: MainActivityViewModel by viewModels()
+    private val viewModel: MainActivityViewModel by viewModels()
 
     override fun onCreate(savedInstanceState: Bundle?) {
         val splashScreen = installSplashScreen()