fs_mgr: add remount command

There is currently no good option for callers to setup overlayfs
on-device, it is automated as part of the adb services.  Add a
remount command that does what is needed that simulates the salient
behaviors of the adb remount command.

Clean up some noise restoring device to original state when done.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: Idf213800a8182cb1c51600c8f574df8a8cd68d4a