Cleanup + refactor gradle files

[tobrun]

Our gradle files are growing large and complex.
I suggest refactoring some tasks into external build scripts:

• javadoc.gradle
• maven.gradle
• make.gradle
• dev_config.gradle
• ...

How to config project using external build script

build.gradle

apply from: 'other.gradle'

other.gradle

println "configuring $project"
task hello << {
    println 'hello from other script'
}

Additional tasks:

• Reevaluate every code block
• Remove Android Plugin Manager Remove sdk-manager-plugin #5930

cc @mapbox/android

[zugaldia]

👍 I really like doing this to my .gradle files and also makes sense from a consistency point of view as we're already following this approach on MAS. E.g.: https://github.com/mapbox/mapbox-java/blob/master/libandroid/lib/build.gradle#L47-L49.

[tobrun]

Ah now I remember where I had seen that before, 🙇 @zugaldia

[tobrun]

Moving of the milestone, no requirement