adbd: switch to su domain when running as root

When adbd runs as root, it should transition into the
su domain. This is needed to run the adbd and shell
domains in enforcing on userdebug / eng devices without
breaking developer workflows.

Introduce a new device_banner command line option.

Change-Id: Ib33c0dd2dd6172035230514ac84fcaed2ecf44d6
diff --git a/rootdir/init.rc b/rootdir/init.rc
index a66a2e4..a32366c 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -442,7 +442,7 @@
     start console
 
 # adbd is controlled via property triggers in init.<platform>.usb.rc
-service adbd /sbin/adbd
+service adbd /sbin/adbd --root_seclabel=u:r:su:s0
     class core
     socket adbd stream 660 system system
     disabled