remount: Simplify fs_mgr_overlayfs_setup.

The use of errno in this function is very difficult to reason about, and
leads to a lot of complexity (eg saving and restoring errno on a case by
case basis).

This CL adds explicit logging in error paths and simplifies the return
state to "succeeded" or "failed".

In addition, the "change" outparam has been simplified as well.
Previously it indicated that *anything* in the filesystem changed. This
is not super useful since the only thing callers care about is whether
or not overlayfs went from "disabled" to "enabled". The outparam now
reflects that.

Bug: 241179247
Test: remount
Change-Id: I5a2b4dcc942e6807c9965cd484de152b47022c4e