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

.gitignore

@@ -7,6 +7,8 @@ website/build
 build/
 yarn-error.log
 
+android/third-party/external/
+
 # Android / Intellij
 local.properties
 *.iml