[shared_preferences] replace AsyncTask by Executor and Guava Future

[hoc081098]

Fixes flutter/flutter#71595
Fixes flutter/flutter#74039

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[SirusCodes]

I already made a PR🤔
#3481

[nyarian]

not sure if guava dependency is worth it here, especially when it is all implementable with plain a ExecutorService instance

[stuartmorgan-g]

Closing in favor of the existing PR at #3481. Please comment there if you have feedback on differences between your approach and that one.