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

xplat/build.gradle

@@ -26,7 +26,10 @@ android {
     }
 
     dependencies {
+        implementation project(':third-party')
         implementation project(':rsocket')
         implementation project(':folly')
     }
 }
+
+preBuild.dependsOn(tasks.getByPath(':third-party:prepare'))