Use SLF4J API's in player module instead of Log4J

[mitschwimmer]

Originally I just asked for the lib-module to utilize SLF4J. But as the player module is now intended to be used on Android it should use Android-friendly logging as well.
The way it is done should allow to keep LOG4J as default. In case the runtime is Android one will need to force exclude LOG4J from the dependencies in gradle and add a SLF4J compatible wrapper for the Android Logging System.

Remarks

1. Line 158 in DeviceStateHandler - Lambdas were used as parameters for the log call. This seems not supported with SLF4J therefore I referenced the paramters straight.
2. Line 246 in DeviceStateHandler - I assumed you intended to log the first log message variant in case of level TRACE and translated to SLF4J API accordingly.
3. I can't avoid merge commits with github it seems. I hope that is ok

[mitschwimmer]

Hm, the build step "import of GPG secrets" fails. I assume this is a permission thing?

[devgianlu]

Hm, the build step "import of GPG secrets" fails. I assume this is a permission thing?

Yep, now fixed.

Line 158 in DeviceStateHandler

That particular call is expensive, just added a check on the log level.