Prepare user storage just before using it.

Wire up preparing of user-specific app storage to existing user
lifecycle hooks.  This way we're sure the storage is ready to roll
just before we start reconciling app data directories.

This also has the nice property that we only prepare storage when
we know that keys are unlocked.

Bug: 25796509
Change-Id: Ic7df9ddbcfb1e20649d11b6cf68d424e3c365ee1