init: Run restorecon_recursive asynchronously

restorecon_recursive may take a long time if there are a lot of files on
the volume.  This can trigger a watchdog timeout in any process that
tries to set a property while it is running.  Fix this by running
restorecon_recursive in its own process.

See https://jira.lineageos.org/browse/BUGBASH-555

Change-Id: I2ce26ff2b5bfc9a133ea42f4dbac50a3ac289c04