fs_mgr: overlay: support non-DAP A/B devices

Loosen some tests and borrow logic from retrofit Dynamic Android
Partitions (DAP) A/B devices.  In non-DAP A/B device case
/mnt/scratch is used, backed by the system other partition.

Tested on taimen with CONFIG_OVERLAY_FS added to its pre 4.6 kernel,
and BOARD_EXT4_SHARE_DUP_BLOCKS := true added to board config. Also
tested on a retrofit DAP A/B device and an untouched taimen.

Test: adb_remount_test.sh
Bug: 120448575
Change-Id: I72aaf4d68db1c3ac380aba425346ab6443cadad3