Merge "fs_mgr: Add OverlayMountPoints() to get the list of overlayfs backing storage" am: ec1b09fca9

Original change: https://android-review.googlesource.com/c/platform/system/core/+/1585166

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I1024f9aeaa46d954f07ed3660b36314f76eca908