[Code health] Don't allow saving a duplicate vertex during polygon drawing session

[shobhitagarwal1612]

Towards #3679

Separate committed and tentative vertices in PolygonDrawingSession. This allows the persistence layer to safely store only the committed vertices without having to assume that the last vertex is user committed vertex or not.

Changes verified using unit tests and testing the flow on a physical device.

@andreia-ferreira PTAL?

[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.09%. Comparing base (1126ebe) to head (45f6ba4).

Files with missing lines	Patch %	Lines
...lection/tasks/polygon/PolygonDrawingSessionImpl.kt	88.88%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3750      +/-   ##
============================================
+ Coverage     68.06%   68.09%   +0.02%     
- Complexity     1618     1619       +1     
============================================
  Files           369      369              
  Lines          9542     9554      +12     
  Branches       1246     1248       +2     
============================================
+ Hits           6495     6506      +11     
  Misses         2377     2377              
- Partials        670      671       +1     

Files with missing lines	Coverage Δ
...acollection/tasks/polygon/DrawAreaTaskViewModel.kt	82.00% <100.00%> (-0.32%)	⬇️
...acollection/tasks/polygon/PolygonDrawingSession.kt	100.00% <ø> (ø)
...lection/tasks/polygon/PolygonDrawingSessionImpl.kt	92.98% <88.88%> (-0.77%)	⬇️

... and 1 file with indirect coverage changes

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