init: refactor to allow successive device_init calls

device_init opens the uevent socket and sehandle when called.
For early_mount however, depending on the fs_mgr flags we may call this
in order to run coldboot for device mapper, dm-verity devices etc.
So the change makes sure we don't try to re-open the uevent socket,
file context handle and selinux status on successive calls to
device_init from within the same process.

b/27805372

Test: Boot saifish successfully

Change-Id: Ifa0e665403211684183efb9be66e4e8d0d86a206
Signed-off-by: Sandeep Patil <sspatil@google.com>