af: Use Application Context for Credentials Manager

[poovamraj]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality - N/A - Changes are internal

📋 Changes

We are using applicationContext to fetch the credentials instead of Activity which is not required and could cause issues when credentials are fetched from outside an activity

📎 References

#233

🎯 Testing

We have updated the existing Unit Tests and added check that proper context is set.

[codecov]

Codecov Report

Patch coverage: 80.00% and project coverage change: -0.06 ⚠️

Comparison is base (d72bf76) 96.14% compared to head (59cbf46) 96.08%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #289      +/-   ##
============================================
- Coverage     96.14%   96.08%   -0.06%     
- Complexity       85       86       +1     
============================================
  Files            97       97              
  Lines          1609     1611       +2     
  Branches        331      331              
============================================
+ Hits           1547     1548       +1     
  Misses           49       49              
- Partials         13       14       +1     

Flag	Coverage Δ
auth0_flutter	100.00% <ø> (ø)
auth0_flutter_android	96.55% <80.00%> (-0.19%)	⬇️
auth0_flutter_ios	99.84% <ø> (ø)
auth0_flutter_platform_interface	87.03% <ø> (ø)

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

Impacted Files	Coverage Δ
...th0_flutter/CredentialsManagerMethodCallHandler.kt	90.00% <75.00%> (-4.74%)	⬇️
...tlin/com/auth0/auth0_flutter/Auth0FlutterPlugin.kt	72.97% <100.00%> (+0.75%)	⬆️

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