Add instructions to download the Gradle wrapper to FGP readme, and add to gitignore (#162332)

Fixes https://github.com/flutter/flutter/issues/161480.

Ended up going with just providing instructions instead of checking in
the binary, as the instructions is just one line.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Co-authored-by: Gray Mackall <mackall@google.com>

dev/bots/analyze.dart

@@ -1742,7 +1742,7 @@ class Hash256 {
 // We have a policy of not checking in binaries into this repository.
 // If you are adding/changing template images, use the flutter_template_images
 // package and a .img.tmpl placeholder instead.
-// If you have other binaries to add, please consult Hixie for advice.
+// If you have other binaries to add, please consult johnmccutchan for advice.
 final Set<Hash256> _legacyBinaries = <Hash256>{
   // DEFAULT ICON IMAGES
 
@@ -2099,7 +2099,7 @@ Future<void> verifyNoBinaries(String workingDirectory, {Set<Hash256>? legacyBina
   // We have a policy of not checking in binaries into this repository.
   // If you are adding/changing template images, use the flutter_template_images
   // package and a .img.tmpl placeholder instead.
-  // If you have other binaries to add, please consult Hixie for advice.
+  // If you have other binaries to add, please consult johnmccutchan for advice.
   assert(
     _legacyBinaries
             .expand<int>((Hash256 hash) => <int>[hash.a, hash.b, hash.c, hash.d])