isolated_app: grant append on app_data_file

isolated_app can already write to a file. Apps may want to append
instead of write.

Fixes: 30984610
Change-Id: I7a90b3311dcaff597f07930ceea0a23b29b0df2d

isolated_app.te

@@ -13,7 +13,7 @@ type isolated_app, domain;
 app_domain(isolated_app)
 
 # Access already open app data files received over Binder or local socket IPC.
-allow isolated_app app_data_file:file { read write getattr lock };
+allow isolated_app app_data_file:file { append read write getattr lock };
 
 allow isolated_app activity_service:service_manager find;
 allow isolated_app display_service:service_manager find;