Add seal if ashmem-dev is backed by memfd

Need to seal the buffer size in align with ashmem if set to PROT_READ
only to prevent untrusted remote process to shrink the buffer size and
crash it.

Bug: 294609150
Test: build
Ignore-AOSP-First: Security
Change-Id: I9288cf30b41e84ad8d3247c204e20482912bff69