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