[connectivity] Fixes lint error by using getApplicationContext()

[josh-burton]

Description

When running Android Lint on a project using the connectivity plugin, Lint gives the following error:

The WIFI_SERVICE must be looked up on the Application context or memory will leak on devices < Android N. Try changing registrar.context() to registrar.context().getApplicationContext()

This applies the suggested change.

Related Issues

flutter/flutter#28928

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[collinjackson]

Looks good. Since the Dart API didn't change and plugin is pre-1.0, I'm making this a +1 change.