Update timezone handling for time only data

[rikroe]

Proposed change

Updates handling for time-only data when returned as da datetime object (currently only charging_time_start).
It is now created without timezone (tzinfo=None) and only works correctly if both the current timezone of the computer and the car are the same.

Unfortunatly the BMW API does not seem to provide any additional data to figure out which timezone is used for these values.

Time-only data where we have a number of minutes remaining (i.e. 250 seconds of climatization) are not impacted, as we can deterministically calculate those.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to this library)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: Incorrect/missing timezons for charging_times #616 (only fixed with HA update)

Checklist

• The code change is tested and works locally.
• Tests have been added to verify that the new code works.