Fix add-on memory calculation

[agners]

Proposed change

Docker versions newer than 19.03 calculate memory usage sligthly
different compared to previous versions. It seems the field
"total_inactive_file" was not available in 19.03, so it can be used
as indicator.

See: https://docs.docker.com/engine/reference/commandline/stats/#description

Essentially, this includes also tmpfs usage in memory usage. Some
more detailed discussions can be found here:
moby/moby#40727

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• 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:
• Link to documentation pull request:
• Link to cli pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints of add-on configuration are added/changed:

• Documentation added/updated for developers.home-assistant.io