ANDROID: ashmem: Add toggle to ignore requests to deny PROT_READ mappings

Memfd does not support preventing a file from being mapped with PROT_READ,
as ashmem does. It would be useful to expose a knob to userspace to
change ashmem's behavior to match memfd to see if any issues arise
during tests.

Therefore, expose a tunable that userspace can use to cause ashmem to
ignore requests to deny PROT_READ mappings.

Bug: 111903542
Change-Id: Id4d1770e93a4fd5a6b3be04fd82c67d0eff0200e
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>