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We now have separate methods for key creation/destruction and
unlocking/locking.  Key unlocking can pass through an opaque token,
but it's left empty for now.

Extend user storage setup to also create system_ce and user_de
paths.  Bring over some path generation logic from installd.

Use strong type checking on user arguments.

Bug: 22358539
Change-Id: I00ba15c7b10dd682640b3f082feade4fb7cbbb5d