Expose global proguard flags for use by R8.

Bug: 293304784
Test: CI
Change-Id: I0e30f3e1affc2493cb63cb94266ad0a6b636ebc8

core/BUILD.bazel

@@ -1,4 +1,28 @@
+# Copyright (C) 2023 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License
+
 # Export tradefed templates for tests.
 exports_files(
     glob(["*.xml"]),
 )
+
+# Export proguard flag files for r8.
+filegroup(
+    name = "global_proguard_flags",
+    srcs = [
+        "proguard.flags",
+        "proguard_basic_keeps.flags",
+    ],
+    visibility = ["//visibility:public"],
+)