[Android]: Prepare for modularization of c/b/banners.

This creates an empty BUILD.gn file downstream can
depend on.

TBR: nyquist
Bug: 1135551
Change-Id: Ia283c64f5c37b9e7afc245a5fc27170dc91c2d71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506179Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822114}

chrome/browser/banners/android/BUILD.gn

+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/android/rules.gni")
+
+# We can't use android_library here since gn doesn't allow those with empty
+# sources. Use java_group until we have files in this module.
+java_group("java") {
+  # Empty module for a three sided patch.
+}