Fix download caching

Summary:
Not sure why, but if we stick this in the build folder,
the artifacts get downloaded every single time despite `onlyIfNewer`.

Reviewed By: jknoxville

Differential Revision: D14132383

fbshipit-source-id: 464e4a6e474877c16a06a6e83ccd1a2e355cbea2

android/third-party/native.gradle

@@ -7,7 +7,7 @@ import org.apache.tools.ant.filters.ReplaceTokens
 final def CACHE_REVISION = 3
 
 final def externalDir = new File("$projectDir/external")
-final def downloadsDir = new File("$buildDir/downloads")
+final def downloadsDir = new File("$externalDir/downloads")
 final def revisionFile = new File("$buildDir/external_cache_revision.txt")
 
 final def getDownloadFileName = { final URL src ->
@@ -25,6 +25,7 @@ final def isCacheOutOfDate = { final Integer revision ->
 }
 
 task createNativeDepsDirectories {
+    buildDir.mkdirs()
     downloadsDir.mkdirs()
     externalDir.mkdirs()
 }