sdcard: Use AID_ROOT constant

To be consistent with the other parts of the code,
and to make it easy to use a differnt value when running in a container.

Bug: 69142822
Test: build
Change-Id: I9b2c7744e3867bb15511dd4ddd7bce281d758a7b

sdcard/fuse.cpp

@@ -323,7 +323,7 @@ static bool check_caller_access_to_name(struct fuse* fuse,
 
     /* Root always has access; access for any other UIDs should always
      * be controlled through packages.list. */
-    if (hdr->uid == 0) {
+    if (hdr->uid == AID_ROOT) {
         return true;
     }