362 increase minimum api level to android 7 (bitfireAT/davx5#363)

* Increase minSdkVersion to 24 (Android 7.0)

* Remove obsolete api level checks

* Use latest dnsjava

* Use latest apache commons

* Minor formatting

* Unify getSystemService() calls

* Remove further unnecessary calls

* Remove noinspection GradleDependency for Apache Commons libs

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Co-authored-by: Ricki Hirner <hirner@bitfire.at>