fs_mgr: Split fs_mgr_overlayfs into two separate files.

One of the paint points for fs_mgr_overlayfs is that mounting and
scratch management code are somewhat unrelated but very intertwined in
the same file.

Split it into two files: fs_mgr_overlayfs_mount, which is only for
mount-related code, and fs_mgr_overlayfs_control, which is only for
setup/teardown code.

The code removed from fs_mgr_overlayfs_control.cpp is code that moved
to fs_mgr_overlayfs_mount.cpp.

This converts a bunch of functions to "static", and splits
fs_mgr_priv_overlayfs.h into two new files (fs_mgr_overlayfs_mount.h and
fs_mgr_overlayfs_control.h).

Bug: N/A
Test: remount
Change-Id: I83da43652b4787f344da75a1d30177df1d7f63b2