GD HCI: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: I708035a21b43bcdef80ef45644205648b08cd074