GD:Wait channel ready after stop/start server

When running multiple tests, the second test will always fail due to
client send rpc request before channel ready. Add wait_channel_ready
function to GdDeviceBase and use it to check channel ready when each
testcase start.

Bug: 136577370
Test: run run_cert.sh
Change-Id: I210dcd69d6e40a315710ba2b8e9e8819e462b42b