DO NOT MERGE. No direct Uri grants from system.

The system should never be extending Uri permission grants from
itself, since it automatically holds all the permissions.  Instead,
the system should always be a mediator between two specific app, and
it should be using startActivityAsCaller() if it needs to extend
permissions.

Blocking at this level fixes an entire class of confused deputy
security issues.

Test: builds, normal intent resolution UI works
Bug: 33019296, 32990341, 32879915, 32879772
Change-Id: Iaa57c393a386d8068e807d0dd0caccc89d8a11db

services/core/java/com/android/server/am/ActivityManagerService.java

@@ -7436,7 +7436,12 @@ public final class ActivityManagerService extends ActivityManagerNative
 
         // Third...  does the caller itself have permission to access
         // this uri?
-        if (UserHandle.getAppId(callingUid) != Process.SYSTEM_UID) {
+        final int callingAppId = UserHandle.getAppId(callingUid);
+        if ((callingAppId == Process.SYSTEM_UID) || (callingAppId == Process.ROOT_UID)) {
+            Slog.w(TAG, "For security reasons, the system cannot issue a Uri permission"
+                    + " grant to " + grantUri + "; use startActivityAsCaller() instead");
+            return -1;
+        } else {
             if (!checkHoldingPermissionsLocked(pm, pi, grantUri, callingUid, modeFlags)) {
                 // Require they hold a strong enough Uri permission
                 if (!checkUriPermissionLocked(grantUri, callingUid, modeFlags)) {