fs_mgr: Add fs_mgr_overlayfs_is_setup()

Provide a means to determine if overlayfs is currently active with
fs_mgr_overlayfs_is_setup().  Search for current clues in the system
that the overrides are in play.  If /mnt/scratch/ is mounted, that
may be enough of a smoking gun because the resource itself may
interfere with operations.

Related to update_engine calling this function to reject performing
an OTA as its operations are incompatible with the overrides.

Test: adb-remount-test.sh
Bug: 120034852
Change-Id: I641163b1559c1a83fe7d0c336d04c10e9956ad3a