Refine the first stage mount for DSU

* Open the ImageManager according to the saved DSU slot.
* Load the saved install_dir when it presents. This is required
  when installing to an external storage and the current code
  seems to break it.
* Remove an unnecessary C++ using.

Test: gsi_tool install
Bug: 144247097

Change-Id: I5b7be25787f235518a25abacefa612c33861f6ef