Move sensor constants and define public names from this module

[frenck]

Proposed change

Makes better use of the const file in the sensor entity component, and defines __all__ to define its public names.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• 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:
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Checklist

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[home-assistant]

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