Fix sample app edge-to-edge layout

[KAbaidoo]

Description

Enable edge-to-edge display in the sample app and apply safe drawing insets to its shared Compose scaffold and splash screen. This keeps the top app bar and screen content visible below the status bar on edge-to-edge devices. Remove the splash activity's obsolete inset listener.

No linked issue was provided.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / maintenance

Testing

• ./gradlew :sample:testDebugUnitTest :app:assembleDebug :sample:spotlessCheck — passed.
• ./gradlew build — blocked by 14 pre-existing :sample:lintDebug errors in untouched Compose preview and test files (FlowOperatorInvokedInComposition and UnusedMaterialScaffoldPaddingParameter).
• Device or emulator validation was unavailable in this environment.

Checklist

• I have read the Contributing guidelines.
• The code builds and existing tests pass (./gradlew build); see the lint limitation above.
• I have added or updated tests where relevant; the existing sample unit tests pass, and no new unit test is needed for system bar placement.
• I have updated the documentation where relevant; this UI-only fix needs no documentation change.
• No secrets, tokens, or subscription keys are included in this change.

[KAbaidoo]

Superseded by #121, which uses the fix/demo-edge-to-edge branch name.