Rename CommandAction.getEnvironment, add AbstractAction.getEnvironment

This is a cleanup to clarify the code.

1. The getEnvironment method in the CommandAction interface does not have
   access to the clientEnv, so it's return value is necessarily incomplete.
   Rename to getIncompleteEnvironmentForTesting.
2. Add a final getEnvironment method to AbstractAction, which returns the
   ActionEnvironment, which is intended to be a complete description of the
   intended final environment of the action (technically, of any spawn running
   within the action). This is not currently used, but is provided to prevent
   action subclasses to add such a method (it may be used in the future).

PiperOrigin-RevId: 196991091