Revert "ashmem: Ensure all memfds have non-executable permissions by default"

This reverts commit ee7a7137.

Reason for revert: We would like to evaluate switching libcutils to
memfd against older kernels that don't support MFD_NOEXEC_SEAL, so
temporarily revert it. MFD_NOEXEC_SEAL will be brought back when
testing has concluded.

Bug: 401214539
Bug: 111903542
Change-Id: I2cf76e3545ac46a65b54889f9030ad44cb1e4d27