I think the issue may be that you are using
<widget id="com.example.app" version="0.0.1" xmlns="http://www.w3.org/ns/widgets" xmlns:cdv="http://cordova.apache.org/ns/1.0" >
(because you created the project using cordova and not phonegap)
which is correct for a cordova project but maybe not for a phonegap build project (and not allow plugins).
try with
<widget id="com.example.app" version="1.0.0" xmlns="http://www.w3.org/ns/widgets" xmlns:gap="http://phonegap.com/ns/1.0">
Important part is xmlns:gap="http://phonegap.com/ns/1.0"
instead of xmlns:cdv="http://cordova.apache.org/ns/1.0"
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