Add installd command to create /data/misc user dir

New command 'mkuser <id>' sets up a user directory in /data/misc/user/,
readable by all apps within that user and writeable by the local system
process.

Change-Id: I5ddde8c4a80f606e723bb41aa64581a0720842d5
Merge "Add installd command to create /data/misc user dir"
auto-merger: merge in from aosp/master