libcutils: Add "daemon" and "bin" users for testing only

Multiple LTP tests require a "daemon" or "bin" user.  These user ids
have been defined since UNIX incept, and even up to the '80s remained
in many of the tools as hard coded values.  Add these two ids with
a cautionary note.

Test: compile
Bug: 31152327
Bug: 31226046
Bug: 32385889
Change-Id: Ida2fb6d817b8ada0624870439fcf848667b31fb3

libcutils/include/private/android_filesystem_config.h

@@ -55,6 +55,9 @@
  */
 
 #define AID_ROOT 0 /* traditional unix root user */
+/* The following are for LTP and should only be used for testing */
+#define AID_DAEMON 1 /* traditional unix daemon owner */
+#define AID_BIN 2    /* traditional unix binaries owner */
 
 #define AID_SYSTEM 1000 /* system server */