Update installd to allow multiuser

* Add ability to select different personas to generate the path to be
  created.

* Move hardcoded paths to read from init's set environment.

* Add unit tests for all the utility functions that build strings to
  make sure they're correct.

* Fill in persona with "0" all the time now. Will be plumbed through in
  later CL.

Change-Id: I0a7f6e3640cb6b052f8823080886ee79e90b679f