Execute task before Android Gradle build?

You can do it in this way:

task build << {
    println 'build'
}
task preBuild << {
    println 'do it before build'
}
build.dependsOn preBuild

Thanks to that task preBuild will be automatically called before build task.

If you want to run preBuild in configuration phase (previous example run preBuild in execution phase) you can do it in this way:

task build << {
    println 'build'
}
build.doFirst {
    println 'do it before build'
}

More about gradle build lifecycle can be read here http://www.gradle.org/docs/current/userguide/build_lifecycle.html.