`get_qcs_objects_for_notebook` needs cleanup and bugfixes

Description of the issue get_qcs_objects_for_notebook was added in #4286 and is super useful to abstract out the authentication logic for accessing QCS. However, I think there are a few rough edges – from a user perspective, I haven’t dug into the technical details yet.

1. In the getting started with QCS tutorial, the object is used for authentication and verification that the authentication went through (device_sampler.is_signed_in) but after that, there’s no usage of the device_sampler object. This doesn’t look like the intended usage of an object specifically has a device and sampler objects ?
2. It looks like sqrt_iswap gateset string is hardcoded deep inside the get_qcs_objects_for_notebook method. This also looks like a bug and something that shouldn’t be intended? https://github.com/quantumlib/Cirq/blob/6562a0521ffda79c8f140774ce292cace346301b/cirq-google/cirq_google/engine/qcs_notebook.py#L119

Cirq version 0.13.0

[dstrain115]

I’m taking this on in #5759

[maffoo]

I think we can address some of the shortcomings of get_qcs_objects_for_notebook in #5240. @verult, should I assign this issue to you?