adb: make adb root not wait-for-device.

`adb root` would hang for devices connected over TCP/IP, because they
never come back. Unfortunately, there doesn't seem to be an easy way to
find out what we're talking to, so replace it with a long sleep for now.

Bug: http://b/27836058
Change-Id: Icd6e6bf817ca9e2124c2961665c37b2f31f7ef29