use add experiment methods (Java and Python)

[kennknowles]

Python:
Convert instances of experiments.append(...)
to debug_options.add_experiment(...)

Java:
Use ExperimentalOptions.addExperiment(...)
instead of getExperiments(), modify, setExperiments() pattern.

Imported from Jira BEAM-6668. Original Jira may contain additional context.
Reported by: udim.

[ash6898]

.take-issue