fix(datastore): configure function triggers initial sync unexpectedly

[HuiSF]

Issue #, if available: #736

Description of changes:

Originally below code is executed within DataStore.configure.

_channel.invokeMethod('setUpObserve', {})

This method channel call invokes observe() method of native DataStore plugins, which triggers API_SYNC. This behavior is undesired especially when data model authorization requires valid user session.

Moving the method call to each DataStore operation process, the actually logic of setUpObserve will only be invoked once.

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[fjnoyp]

Looks good! Just need to wait for CircleCI to be unblocked again.

[codecov-commenter]

Codecov Report

Merging #986 (22d861d) into main (3461561) will decrease coverage by 0.01%.
The diff coverage is 63.63%.

@@             Coverage Diff              @@
##               main     #986      +/-   ##
============================================
- Coverage     51.38%   51.37%   -0.02%     
  Complexity        1        1              
============================================
  Files           325      325              
  Lines         12312    12321       +9     
  Branches        460      461       +1     
============================================
+ Hits           6327     6330       +3     
- Misses         5821     5826       +5     
- Partials        164      165       +1     

Flag	Coverage Δ
android-unit-tests	62.20% <0.00%> (-0.08%)	⬇️
flutter-unit-tests	37.30% <87.50%> (+<0.01%)	⬆️
ios-unit-tests	95.41% <ø> (ø)

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Impacted Files	Coverage Δ
...mplify/amplify_datastore/AmplifyDataStorePlugin.kt	66.94% <0.00%> (-0.57%)	⬇️
...mplify_datastore/lib/method_channel_datastore.dart	73.91% <87.50%> (+0.51%)	⬆️
...kages/amplify_datastore/lib/amplify_datastore.dart	68.29% <0.00%> (-4.88%)	⬇️