[google_sign_in] Convert Android to Pigeon

Replaces the direct method channel implementation with Pigeon.

Since `google_sign_in`, unlike most of our plugins, exposes an API
that's intended for direct cross-plugin native use, the existing methods
are all left in place, but refactored as passthroughs to the new Pigeon
versions. To ensure that they aren't broken, the existing Java tests are
preserved unchanged (as a "legacy" copy) with `onMethodCall` left in
place for now just to allow the tests to continue to run as-is. Since
that dispatches to the legacy methods, this keeps the existing coverage
of those methods.

The new tests are a copy of the legacy tests, minimally translated to
use the new Pigeon variants, to ensure continuity of testing to the new
version.

Part of flutter/flutter#117908

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 6.1.17
+
+* Converts method channels to Pigeon.
+
 ## 6.1.16
 
 * Updates Guava to version 32.0.1.