Use application context to get system service

[katjajohanna]

This fixes the exception when building the application:

react-native-android-wifi/android/src/main/java/com/devstepbcn/wifi/AndroidWifiModule.java:49: Error: The WIFI_SERVICE must be looked up on the Application context or memory will leak on devices < Android N. Try changing reactContext to reactContext.getApplicationContext()  [WifiManagerLeak]
  wifi = (WifiManager)reactContext.getSystemService(Context.WIFI_SERVICE);
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   Explanation for issues of type "WifiManagerLeak":
   On versions prior to Android N (24), initializing the WifiManager via
   Context#getSystemService can cause a memory leak if the context is not the
   application context. Change context.getSystemService(...) to
   context.getApplicationContext().getSystemService(...).