[shared_preferences_foundation] Add Swift runtime search paths for Objective-C apps

[jmagman]

shared_preferences_foundation is using Swift as of #6940. The Objective-C projects do not automatically set up the search paths to find the Swift runtimes. Hook that up in the shared_preferences_foundation podspec.

See explanation at flutter/flutter#16049 (comment) and the first time this was done in plugins: #6369

Convert the example project back to Objective-C to catch future versions of this issue.

Fixes flutter/flutter#118418

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