Keep OEM defined DisplayAreaPolicy.Provider

Without this change, it may throw ClassNotFoundException when OEMs
define their own DisplayAreaPolicy.Provider.

Fix: 235942308
Test: verify with a customized provider
Change-Id: I397f4fc05ecd5732e3ebb417de3a66c280f678b3
diff --git a/services/proguard.flags b/services/proguard.flags
index bad02b4..c648f7d 100644
--- a/services/proguard.flags
+++ b/services/proguard.flags
@@ -68,6 +68,9 @@
 # TODO(b/210510433): Revisit and consider generating from frameworks/base/core/res/res/values/config.xml.
 -keep,allowoptimization,allowaccessmodification public class com.android.server.notification.** implements com.android.server.notification.NotificationSignalExtractor
 
+# OEM provided DisplayAreaPolicy.Provider defined in frameworks/base/core/res/res/values/config.xml.
+-keep,allowoptimization,allowaccessmodification class com.android.server.wm.** implements com.android.server.wm.DisplayAreaPolicy$Provider
+
 # JNI keep rules
 # TODO(b/210510433): Revisit and fix with @Keep, or consider auto-generating from
 # frameworks/base/services/core/jni/onload.cpp.