fs_mgr: Add OverlayMountPoints() to get the list of overlayfs backing storage

Remove kOverlayMountPoints; add OverlayMountPoints().

Fix a regression where if host system didn't install a scratch_gsi
partition and physical /cache partition is mounted, then DSU guest
system could use /cache as its overlayfs backing storage. This is
generally unwanted as the /cache partition could be shared between host
and guest.
Dynamically return the list of overlayfs backing storage candidates, so
we don't accidentally fall back to use /cache storage within a DSU
system.

Bug: 165925766
Bug: 179980369
Test: 1. Prepare a DUT that have a physical cache partition, such as
  cuttlefish.
  2. Install a DSU system, and unsure that scratch_gsi is not installed.
  3. Reboot into DSU, verify that adb remount fails and overlayfs
     scratch is not created under /cache.
Change-Id: I1815ac5367c0aac8614aeaabebe0e2cb91cbe161