feat(FocusZone): Set the ref of the Focus Zone in Embed mode

[sophieH29]

Previously, the ref of the FocusZone was set only for Wrap mode.
Current PR aims to set the ref for the Embed mode of the Focus zone too, so we can access FocusZone and its functions directly from the components, no matter what kind of the FocusZone is used.

[codecov]

Codecov Report

Merging #435 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #435   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
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  Files          41       41           
  Lines        1337     1337           
  Branches      193      193           
=======================================
  Hits         1228     1228           
  Misses        105      105           
  Partials        4        4

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