Reorganize native dep download and extraction (#154)

Summary:
This moves the native setup to a separate gradle file and unpacks everything outside the build folder. That way we don't throw everything away on every incremental build.

This is an intermediate step to get to a more declarative setup like Fresco has it and get rid of the individual hacks we have for every download.
Pull Request resolved: https://github.com/facebook/Sonar/pull/154

Reviewed By: jknoxville

Differential Revision: D8833812

Pulled By: passy

fbshipit-source-id: 2424c9d6e22e6092c04af344e06939b4a6aba041

build.gradle

@@ -9,10 +9,13 @@ buildscript {
         classpath "com.github.dcendents:android-maven-gradle-plugin:${ANDROID_MAVEN_GRADLE_PLUGIN_VERSION}"
         classpath "com.github.ben-manes:gradle-versions-plugin:${GRADLE_VERSIONS_PLUGIN_VERSION}"
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${KOTLIN_VERSION}"
-        classpath 'de.undercouch:gradle-download-task:3.1.2'
     }
 }
 
+plugins {
+    id 'de.undercouch.download' version '3.4.3'
+}
+
 subprojects {
     repositories {
         google()