fix(analytics): adds flutter sdk to example apps

[haverchuck]

Description of changes:
Adds the flutter sdk as dependency to analytics example apps.

Fixes the following melos error:

Running "flutter pub get" in workspace packages...
Note: this may take a while in large workspaces such as this one.
  ✓ amplify_analytics_pinpoint
    └> packages/analytics/amplify_analytics_pinpoint
  ✓ amplify_analytics_pinpoint_android
    └> packages/analytics/amplify_analytics_pinpoint_android
  - amplify_analytics_pinpoint_android_example
    └> packages/analytics/amplify_analytics_pinpoint_android/example
    └> Failed to install.

Resolving dependencies...
Because amplify_analytics_pinpoint_android_example requires the Flutter SDK, version solving failed.

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[ragingsquirrel3]

ty!