fix(Embed): get rid of white flash

[lucivpav]

Now, when you activate a video or iframe Embed, no white flash appears. Instead, the placeholder image is used.

Fixes #1586

[codecov]

Codecov Report

Merging #1909 into master will decrease coverage by 0.04%.
The diff coverage is 55%.

@@            Coverage Diff             @@
##           master    #1909      +/-   ##
==========================================
- Coverage    70.4%   70.35%   -0.05%     
==========================================
  Files         892      892              
  Lines        7870     7884      +14     
  Branches     2298     2305       +7     
==========================================
+ Hits         5541     5547       +6     
- Misses       2316     2324       +8     
  Partials       13       13

Impacted Files	Coverage Δ
...t/src/themes/teams/components/Embed/embedStyles.ts	0% <0%> (ø)	⬆️
packages/react/src/components/Embed/Embed.tsx	77.77% <61.11%> (-17.88%)	⬇️

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