[incfs] Use MountRegistry to import existing mounts on start

This is a big cleanup in IncrementalService that makes it behave
nicely on runtime restart, and more:

- fixed a bunch of threading issues in createStorage/bind
- made public functions correctly accept any path in any
  bind mount and translate it to the proper root mount
- got rid of "using namespace" in headers, cleaned includes
- removed all unused functions
- set CLOEXEC bit on all duped FDs

Bug: 151241369
Test: atest PackageManagerShellCommandTest \
  PackageManagerShellCommandIncrementalTest \
  IncrementalServiceTest

Change-Id: Ided4415aabfbfca3925b5e71c91896055886ac4a