xSQLServerAlwaysOnAvailabilityGroup\Replica: Utilize the Get-PrimaryReplicaServerObject helper function

[randomnote1]

Pull Request (PR) description
Updated xSQLServerAlwaysOnAvailabilityGroup and xSQLServerAlwaysOnAvailabilityGroupReplica to utilize the Get-PrimaryReplicaServerObject helper function.

This Pull Request (PR) fixes the following issues:
Fixes #630

Task list:

• Change details added to Unreleased section of CHANGELOG.md?
• Added/updated documentation, comment-based help and descriptions in .schema.mof files where appropriate?
• Examples appropriately updated?
• New/changed code adheres to Style Guidelines?
• Unit and (optional) Integration tests created/updated where possible?

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This change is 

[codecov-io]

Codecov Report

Merging #842 into dev will decrease coverage by <1%.
The diff coverage is 100%.

@@         Coverage Diff         @@
##            dev   #842   +/-   ##
===================================
- Coverage    96%    96%   -1%     
===================================
  Files        31     31           
  Lines      3314   3311    -3     
===================================
- Hits       3197   3194    -3     
  Misses      117    117

[johlju]

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Reviewed 2 of 2 files at r1, 3 of 3 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[johlju]

Awesome work! 😄