overlayfs: fs_mgr_overlayfs_setup() should accept Fstab as input

Right now fs_mgr_overlayfs_setup() always reads the default fstab and
this makes the "-T" option of remount useless.
Change it so that the fstab is passed in by the caller.

Bug: 243501054
Test: adb remount -vT <path/to/fstab>
Test: and check that overlay is active after reboot
Change-Id: Ia4101938a50c305f105c57018b02aec01f862dec