[shared_preferences] Defer SharedPreferences fetch in `LegacySharedPreferencesPlugin` until first use on Android (#13431)

Defers `getSharedPreferences` in `LegacySharedPreferencesPlugin` out of `onAttachedToEngine` until first use, avoiding synchronous disk I/O on the Android main thread during startup.

The following screenshot shows that the time taken for `RegisterFlutterPlugin` significantly decreases with the change, which validates the fix:
<img width="1920" height="552" alt="shared_preferences_registration_comparison_perfetto" src="https://github.com/user-attachments/assets/c1fea032-a499-4fa4-91de-13648ea03999" />

I also previously implemented this in my app, which was later negated by an upgrade of the shared_preferences packages into g3 and caused cold startup latency regression alerts on my app. Hoping to upstream the fix to get this optimization to all Flutter users.

https://github.com/flutter/flutter/issues/195313 will be fixed by this PR.

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