Fix memory leak on android

[Maikell84]

Gradle linter gives a warning:

:lintVitalRelease/home/michael/Projects/smart-mirror/application/platforms/android/src/com/pylonproducts/wifiwizard/WifiWizard.java:57: Error: The WIFI_SERVICE must be looked up on the Application context or memory will leak on devices < Android N. Try changing cordova.getActivity() to cordova.getActivity().getApplicationContext() [WifiManagerLeak]
this.wifiManager = (WifiManager) cordova.getActivity().getSystemService(Context.WIFI_SERVICE);

This PR resolves it.

[tripflex]

Thanks for this, I created a fork of WifiWizard (named WifiWizard2), and added this PR to it, along with credit for you in the changelog:
https://github.com/tripflex/WifiWizard2