[RN] Get guest credentials identity cached into AsyncStorage

[powerful23]

Issue #, if available:
When reloading the app, the guest user will get a new identity id because the previous one is not cached. Fixes #383
Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mlabieniec]

👍

[codecov-io]

Codecov Report

Merging #835 into master will decrease coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master    #835      +/-   ##
=========================================
- Coverage   87.72%   87.7%   -0.02%     
=========================================
  Files          75      75              
  Lines        3593    3604      +11     
  Branches      683     684       +1     
=========================================
+ Hits         3152    3161       +9     
- Misses        419     420       +1     
- Partials       22      23       +1

Impacted Files	Coverage Δ
...kages/aws-amplify/src/Common/RNComponents/index.ts	80% <100%> (+5%)	⬆️
packages/aws-amplify/src/Auth/Auth.ts	88.17% <83.33%> (-0.14%)	⬇️

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[github-actions]

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