Merge "fs_mgr: Split fs_mgr_overlayfs into two separate files." into main am: 3d0a332ff5 am: f7a0081b99

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

Change-Id: Ifb2699c3ce5f273c4bf2ffcf24dbc3ae4d807a0f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>