DatePicker: Fixed Values - Don't hide year / date in toolbar

[steveoandrey]

Description

Resolves visual issue with Date Picker when using Fixed Values.
It was pointed out by @Garderoben in #2990
PR contains the key part of #3490 without adding any new parameters.
Instead of hiding these displays alltogether, it is just made impossible to switch to those views (For example, the button click does not do the action

How Has This Been Tested?

Unit test were adapted to reflect the changed visuals
Manual visual tests were done.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

It is 100% non breaking, because the fixed values feature is not yet released.

Before (dev)

After (PR)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Merging #3538 (1bb690c) into dev (60e5bb8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #3538   +/-   ##
=======================================
  Coverage   90.34%   90.34%           
=======================================
  Files         333      333           
  Lines       10445    10445           
=======================================
  Hits         9437     9437           
  Misses       1008     1008           

Impacted Files	Coverage Δ
...azor/Components/DatePicker/MudBaseDatePicker.razor	97.72% <100.00%> (-0.06%)	⬇️
...r/Components/DatePicker/MudBaseDatePicker.razor.cs	86.54% <100.00%> (+0.07%)	⬆️

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