customizability options

[zworkb]

customizability options for p4a

This PR introduces a set of options which deal with integration of java/kotlin libraries

• android entrypoint: this option already existed in buildozer but was not implemented in p4a.
  this allows to specify a custom class inherited class from org.kivy.android.PythonActivity as entrypoint
• android apptheme: specifies a custom app-theme for the main activity, will be generated into the manifest
• add_compiler_options
• gradle-repository: with this option it is possible to specify additional repositories for the gradle build
• packaging-options: results in packagingOptions in the gradle script, can be necessary to resolve conflicts in java/kotlin packages

this PR also includes changes from #912

this PR also needs kivy/python-for-android#1869

[inclement]

Thanks @zworkb, this looks fine if you're done with it?