Remove underscores from private property and method names

[rachel-fenichel]

Is your feature request related to a problem? Please describe.

Why: to match the current google style guide and as part of checking tests for accesses to private properties.

Describe the solution you'd like

Rename only things that are private or otherwise internal to a file.
Anything with an underscore that is public or protected should be left alone to avoid API churn.

Describe alternatives you've considered

Could also rename protected stuff, but that has to be handled on a case-by-case basis.

Alternatively, we could leave these alone because they aren't part of the external API anyway.