Remove deprecated javacompile

[tobrun]

Starting with Android Studio 3.3.0, we will receive a warning when we perform a gradle sync:

WARNING: API 'variant.getJavaCompile()' is obsolete and has been replaced with 'variant.getJavaCompileProvider()'.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 a stack trace.

Capturing a solution from here. We need to replace the following:

source = variant.getJavaCompiler().source	

with

source = variant.sourceSets.collect { it.java.sourceFiles }.inject { m, i -> m + i }

cc @mapbox/android, this fix needs to be applied to other projects using the same javadoc configuration