Remount tmpfs for overlays for compatibility with old 1st stage

Rather than change how the second stage resources tmpfs is mounted by
default, remount to remove the MS_NOEXEC flag if we are going to run
overlay_remounter from that volume

Test: system/core/fs_mgr/tests/adb-remount-test.sh, OEM confirms fixes 394396860
Bug: 399680970
Bug: 394396860

Change-Id: I5f3e8edd3b0b165eee07f3adc30204291f469995