Use gradle variables for setting the library versions

[jcesarmobile]

Right now we are replacing the library versions on the build.gradle with javascript, but gradle actually supports the variables, so maybe we can create the variables from plugin.xml but let users overwrite them in the build.gradle instead of using plugin.xml or package.json Cordova variables

https://medium.com/@ivan_m/use-variables-to-manage-gradle-android-dependencies-library-version-numbers-92f0e362a2fc

Related PR where we replace the variables: #222

[jcesarmobile]

Related #206

[ionitron-bot]

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