Consistent creation/destruction of user data.

Preparing and destroying users currently needs to be split across
installd, system_server, and vold, since no single party has all the
required SELinux permissions.

Start passing down flags so we only prepare/destroy storage areas
that have keys currently installed.

Add delete_dir_contents_and_dir() argument to ignore ENOENT, since
all we care about is the directory being gone.

Bug: 27896918
Change-Id: Ia1cccc9b35e6ff09e3ca50effeab676157b856e8