Rework app isolate API

[nirinchev]

• Adds App.getById to be used on background isolates.
• Pulls in Expose get_cached_app in C API realm-core#7182 to start constructing cached apps.
• Logs a warning when App(AppConfiguration) is used outside of the main isolate.

Fixes #1433

[coveralls-official]

Pull Request Test Coverage Report for Build 7135071089

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• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 88.406%

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Totals
Change from base Build 7083397893:	0.06%
Covered Lines:	3477
Relevant Lines:	3933

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💛 - Coveralls

[tgoyne]

Obviously I can't judge if the dart code is good but this all looks reasonable to me.

[nirinchev]

Thanks @tgoyne, appreciate the review! I mostly needed a high-level sanity check to make sure this approach is sound and in line with how the Core API are supposed to be used.