Add a rule to keep classes that implements CustomSliceable

Bug: 115862774
Test: visual, SliceViewer
Change-Id: Iff52a61423ef6822430a1f269a897b46d46f5253

proguard.flags

@@ -56,3 +56,8 @@
     public static ** SUMMARY_PROVIDER_FACTORY;
 }
 -keep class androidx.core.app.CoreComponentFactory
+
+# Keep classes that implements CustomSliceable, which are used by reflection.
+-keepclasseswithmembers class * implements com.android.settings.slices.CustomSliceable {
+    public <init>(android.content.Context);
+}