Don't process value adjusting twice

[Bakual]

As reported by @brianteeman if you enabled language debuggin, you will get a fatal error when trying to edit an article in backend.
This is because the getLayoutData method is run both for the getInput and getLabel methods and thus the value is processed twice. Now the first round works fine and leaves the value as **2016-12-15**, but the second round PHP can't work with that value anymore because it's no longer a valid date string.

Summary of Changes

This PR moves the value processing from the getLayoutData method to the getInput one so it is only run once.

Testing Instructions

• Enable language debugging and edit an article in backend. See that the error occurs. (Form looks broken and there is a funny browser title).
• Apply PR and the form works again.
• Make sure the date eg in "Publish Up" is as expected, also taking also into account timezone settings.
• Make sure the translate dateformat works. You need to add language strings to your second language as explained in TranslateFormat in all other forms #13158 (when you're at it, test that one as well :smile)

Documentation Changes Required

None

[infograf768]

I have tested this item ✅ successfully on 4826e6d

Works fine here.

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[anibalsanchez]

I have tested this item ✅ successfully on 4826e6d

Test OK

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[jeckodevelopment]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13233.}