Update API variant.getJavaCompile() usage

[VictorAtPL]

I've got following warning during compilation:

WARNING: API 'variant.getJavaCompile()' is obsolete and has been replaced with 'variant.getJavaCompileProvider()'.
It will be removed at the end of 2019.
For more information, see https://d.android.com/r/tools/task-configuration-avoidance.
To determine what is calling variant.getJavaCompile(), use -Pandroid.debug.obsoleteApi=true on the command line to display more information.

In a spare time, please do update the API.