fix(amplify-flutter): MissingPluginException when android app restarts

[Amplifiyer]

Issue #, if available: fixed #99

Description of changes:

• Swallow exceptions that are thrown in the onAttachedToEngine method as they are never reached to the user and also never even logged.
• Update example app with new exceptions
• Fix AmplifyAlreadyConfiguredException
• Demonstrate in all of our examples how to catch and work around AmplifyAlreadyConfiguredException

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #345 (036edd1) into master (193f36a) will decrease coverage by 0.32%.
The diff coverage is 4.44%.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
- Coverage   69.42%   69.09%   -0.33%     
==========================================
  Files         238      238              
  Lines        7070     7105      +35     
  Branches      312      315       +3     
==========================================
+ Hits         4908     4909       +1     
- Misses       2043     2076      +33     
- Partials      119      120       +1     

Flag	Coverage Δ
android-unit-tests	61.09% <2.70%> (-1.15%)	⬇️
flutter-unit-tests	59.44% <12.50%> (-0.12%)	⬇️
ios-unit-tests	77.32% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...alytics_pinpoint/AmplifyAnalyticsPinpointPlugin.kt	16.66% <0.00%> (-4.03%)	⬇️
.../amazonaws/amplify/amplify_api/AmplifyApiPlugin.kt	55.00% <0.00%> (-6.12%)	⬇️
...zonaws/amplify/amplify_auth_cognito/AuthCognito.kt	58.44% <0.00%> (-1.30%)	⬇️
packages/amplify_core/lib/amplify_core.dart	100.00% <ø> (ø)
...mplify/amplify_datastore/AmplifyDataStorePlugin.kt	60.82% <0.00%> (-4.37%)	⬇️
packages/amplify_flutter/lib/amplify.dart	58.73% <0.00%> (-0.95%)	⬇️
packages/amplify_flutter/lib/amplify_hub.dart	93.75% <ø> (ø)
...mplify_datastore/lib/method_channel_datastore.dart	85.24% <20.00%> (-4.23%)	⬇️
...aws/amplify/amplify_api/rest_api/FlutterRestApi.kt	62.66% <50.00%> (-0.85%)	⬇️

[haverchuck]

LGTM - but check out the android unit tests.

[brunovsiqueira]

Hi there @Amplifiyer. When this fix is going to be released? I am having several crashes in my production app due to this problem.

[Amplifiyer]

@brunovsiqueira, we are in final phases of testing and will release very soon. Stay tuned!