Move android build files to xplat

Summary:
This should work with our existing shipit config. Moves the build
files to xplat and sets up a standard gradle directory structure
that we will continue to move more files into.

Reviewed By: jknoxville

Differential Revision: D8859483

fbshipit-source-id: 5df5882a3bcaee36280af68aeaa740290288e0d3

android/build.gradle

@@ -38,9 +38,6 @@ android {
                 exclude 'console/JavascriptEnvironment.java'
                 exclude 'console/JavascriptSession.java'
             }
-            res {
-                srcDir 'res'
-            }
         }
     }