adb: wait for device to disconnect upon `adb root`.

Previously, there was a race where if adb root took longer than 3
seconds to take effect, we'd return early and allow subsequent commands
to be targeted at the still-not-dead transport, and spuriously fail.

Bug: http://b/124244488
Test: test_device.py
Change-Id: I791a4f82946eb28e4d37729ab0ed3b7fc05b42a2