Don't depend on the androidartifacts plugin but react on it

[StefMa]

Goes hand in hand with #32 #31 and #34 ... somehow

Currently we apply the guru.stefma.artifacts plugin when this plugin got applied:

ArtifactoryPublish/src/main/kotlin/guru/stefma/artifactorypublish/ArtifactoryPublishPlugin.kt

Line 19 in 8c6f4b8

project.pluginManager.apply("guru.stefma.artifacts")

The better approach would be to "react" on the plugin - if its already applied.
So we have a way better separation:

• When the guru.stefma.artifacts plugin is applied (or better some "base plugin" so we can make sure that it is really applied)
• Then we can do our work

Otherwise we do nothing...