Move AutoUpdate reporting to HeartBeat event

[pgombar]

Description

1. AutoUpdate needs to be reported more frequently than just on system startup (Emit AutoUpdate value at service start only #1907) in order to know how many VMs in our fleet are not updating due to potential issues or due to AutoUpdate being set to false.
2. Move AutoUpdate reporting to HeartBeat event which is emitted and logged locally every half an hour.
3. Emit HeartBeat in the beginning of the exthandlers thread.

Local log message:
2020-06-25T21:01:19.140669Z INFO ExtHandler ExtHandler [HEARTBEAT] Agent WALinuxAgent-2.2.48.1 is running as the goal state agent [DEBUG HeartbeatCounter: 1;HeartbeatId: BC244091-DA90-476D-A49D-6184F6A4125D;DroppedPackets: 0;UpdateGSErrors: 0;AutoUpdate: 1]

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[codecov]

Codecov Report

Merging #1919 into release-2.2.49 will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           release-2.2.49    #1919      +/-   ##
==================================================
- Coverage           69.47%   69.46%   -0.02%     
==================================================
  Files                  85       85              
  Lines               11870    11867       -3     
  Branches             1667     1667              
==================================================
- Hits                 8247     8243       -4     
- Misses               3252     3253       +1     
  Partials              371      371              

Impacted Files	Coverage Δ
azurelinuxagent/common/event.py	85.93% <ø> (-0.04%)	⬇️
azurelinuxagent/ga/update.py	87.59% <100.00%> (-0.20%)	⬇️

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[larohra]

1 minor comment everything else LGTM

[larohra]

LGTM