libtimeinstate: open maps write-only

Userspace never reads from these maps, so we can reduce the needed
permissions by opening them write-only

Test: libtimeinstate_test passes
Bug: 138317993
Change-Id: Icbd87b35dc93992e6542ae04ac1ea733b274d28a
Signed-off-by: Connor O'Brien <connoro@google.com>