ANDROID: KVM: Fix ToCToU issue when admitting pages in memcache

Use a local copy of the host provided data to prevent a time
of check time of use when we give pages to the hypervisor
memcache.

Bug: 441436872
Change-Id: I548e2721dc324124cf11f56cd3eb497a8600cdd7
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Signed-off-by: Keir Fraser <keirf@google.com>
(cherry picked from commit 4cfc9c2d)
Signed-off-by: Lee Jones <joneslee@google.com>