ueventd: make selinux labeling optional for device creation

This is to setup a way for us to run coldboot during init first stage
and also at ueventd startup. We do not have all of the file context
during the first stage, so the "early" coldboot needs to proceed without
labelling the device nodes. However, the follow up in ueventd must label
these nodes. This change allows us to do both.

b/27805372

Test: Boot angler successfully and compare do before/after comparison of
      the output of 'ls -AclpqRZ /dev' to ensure there are no differences.

Change-Id: I5e88bd7da8a1d2cc41e3abba30dda463ecbde32e
Signed-off-by: Sandeep Patil <sspatil@google.com>