"Reuse" SharedValues for APK and IPA path

[janpio]

Actions like hockey are using SharedValues::GRADLE_APK_OUTPUT_PATH or SharedValues::IPA_OUTPUT_PATH so you don't have to explicitly set the APK path:

https://github.com/fastlane/fastlane/blob/master/fastlane/lib/fastlane/actions/hockey.rb#L185
https://github.com/fastlane/fastlane/blob/master/fastlane/lib/fastlane/actions/hockey.rb#L204

This plugin could also set these values when a build succeeded additionally to its own variables so hockey and similar actions also don't need to manually get the path.

[janpio]

When #8 is implemented, this can also be done for https://github.com/fastlane/fastlane/blob/master/fastlane/lib/fastlane/actions/hockey.rb#L216