Merge "fs_mgr: Split fs_mgr_overlayfs into two separate files." into main am: 3d0a332ff5

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2656899

Change-Id: I5f6ae67bbdca29882be4bb466d78c66a18f92cbe
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>