[SetupCompat] Add proguard_flag_files for compat library.

Bug: 254388464
Bug: 258123637
Test: manual
Change-Id: I0f568d846c0c58b9f31a7a75bc08c473ec80723d
diff --git a/Android.bp b/Android.bp
index d3a42a7..26a0270 100644
--- a/Android.bp
+++ b/Android.bp
@@ -84,4 +84,7 @@
     ],
     min_sdk_version: "14",
     sdk_version: "current",
+    optimize: {
+        proguard_flags_files: ["proguard.flags"],
+    }
 }