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.

Use the new device_banner command line option.

Change-Id: Ib33c0dd2dd6172035230514ac84fcaed2ecf44d6
minadbd: remove dead code

Change-Id: Ia1f34a17ae582575f8cd3514ed7bc015b0a5006e