[app-mgmt-test] Use gauranteed invalid address

While it is unlikely, with ASLR or an unusual ELF the top page
(previously used as an invalid user address) could be occupied.

We currently forbid mapping pages below 0x8000, so this is more clearly
unmapped, but is neither NULL nor in the kernel range.

Bug: 80146040
Change-Id: I4b915b692fe5f20f4033009cbd7fade8f3b84395
1 file changed