fs_mgr_overlayfs_mount_fstab_entry(): Rename source device name

adb-remount-test.sh would fail if a overlayfs named "overlay" is mounted
in first_stage_init via fs_mgr_overlayfs_mount_fstab_entry():

  [  FAILED  ] overlay takeover unexpected at this phase

Prepend the source device name with "overlay-", so that the overlayfs
mounted by fs_mgr_overlayfs_mount_fstab_entry() mustn't be named
"overlay".

Bug: 188862155
Test: adb-remount-test.sh
Change-Id: Ia0b3f1c92ed5650c3e584fba23758344a4733657