Allow system account to have screenshot permission

This acts as a fallback, and it should be safe to allow system to have
the permission.

Flag: EXEMPT bug fix
Bug: 356080443
Test: manual
Change-Id: If40acd813c2eddaf1d8f496c01ce06473c2c680b
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index b140fd6..340c8bd 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -6907,7 +6907,8 @@
     IPCThreadState* ipc = IPCThreadState::self();
     const int pid = ipc->getCallingPid();
     const int uid = ipc->getCallingUid();
-    if (uid == AID_GRAPHICS || PermissionCache::checkPermission(sReadFramebuffer, pid, uid)) {
+    if (uid == AID_GRAPHICS || uid == AID_SYSTEM ||
+        PermissionCache::checkPermission(sReadFramebuffer, pid, uid)) {
         return OK;
     }