Ensure build dir exists before writing to it (#436)

Summary:
This is because `doLast` may happen after the cleanup came around to undo `createNativeDepsDirectories` specified as dependency.
Pull Request resolved: https://github.com/facebook/flipper/pull/436

Reviewed By: jknoxville

Differential Revision: D15260597

Pulled By: passy

fbshipit-source-id: b752af2b71217ebc1d802b43f1585e858a41fc00

android/third-party/native.gradle

@@ -226,6 +226,7 @@ task prepareRSocket(dependsOn: [downloadRSocket], type: Copy) {
 
 task writeCacheRevision(dependsOn: createNativeDepsDirectories) {
     doLast {
+        buildDir.mkdirs()
         revisionFile.text = CACHE_REVISION.toString()
     }
 }