Emit AutoUpdate value at service start only

[narrieta]

While debugging the agent I noticed we emit that event in each iteration of the main loop. Moving it to service initialization.

[codecov]

Codecov Report

Merging #1907 into develop will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop    #1907      +/-   ##
===========================================
+ Coverage    69.37%   69.38%   +0.01%     
===========================================
  Files           85       85              
  Lines        11811    11813       +2     
  Branches      1658     1658              
===========================================
+ Hits          8194     8197       +3     
+ Misses        3249     3248       -1     
  Partials       368      368              

Impacted Files	Coverage Δ
azurelinuxagent/ga/update.py	87.78% <66.66%> (+0.19%)	⬆️

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