fs_mgr: overlayfs support legacy devices (marlin) Part Deux

On legacy devices system_<other> partition is blocked from
becoming the backing store under certain circumstances.

Test: system/core/fs_mgr/tests/adb-remount-test.sh
Bug: 120448575
Bug: 123079041
Change-Id: I1803f072ca21bc116554eee1d01a1dbd2c9ed0c9
3 files changed