[native] Don't always download Folly during Android build

Summary:
From the [documentation](https://github.com/michel-kraemer/gradle-download-task#download-task) for the `Download` task:

> By default, the plugin always performs a download even if the destination file already exists. If you want to prevent files from being downloaded again, use the `overwrite` flag

I had copied [this configuration](karol-bisztyga/rnfbjni@704994b) from @karol-bisztyga's test repo, but React Native sets `overwrite(false)`. I don't think we need to download it every time.

Test Plan: Compile Android and make sure `downloadFolly` doesn't get re-run

Reviewers: karol-bisztyga, atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D1230

native/android/app/build.gradle

@@ -145,7 +145,7 @@ task createNativeDepsDirectories {
 task downloadFolly(dependsOn: createNativeDepsDirectories, type: Download) {
     src("https://github.com/facebook/folly/archive/v${FOLLY_VERSION}.tar.gz")
     onlyIfNewer(true)
-    overwrite(true)
+    overwrite(false)
     dest(new File(downloadsDir, "folly-${FOLLY_VERSION}.tar.gz"))
 }