Do not list adb devices when a device id is given

Summary:
On Android, this method gets called twice: first when establishing
a host-server connection, then when attaching to a process id.

Each call takes several seconds to finish (especially slower on Windows)
and eliminating the call for the typical case improves latency significantly.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D79586