Add last clean times to xiaomi vacuum

[rytilahti]

Description:

Add start and stop times of the last cleaning, useful for displaying or scheduling based on their value. The keys are the same as used by neato platform:

clean_start: 2018-12-03T14:15:16
clean_stop: 2018-12-03T14:39:19

Continuation of #17558 (can't reopen after a forced push). This is ready to be merged when #19042 gets merged.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
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If the code does not interact with devices:

• Tests have been added to verify that the new code works.