Unify ble manager for silabs platforms

[arun-silabs]

Summary

This PR refactors the BLEManager implementation for the Silabs platform to reduce code duplication between the efr32 and rs911x variants.

• Introduced AbstractBLEManagerImpl to consolidate shared BLE manager state, initialization, and delegate helpers. This centralizes common logic, preventing its duplication across different platform implementations.
• Updated the efr32 and rs911x BLEManagerImpl classes to inherit from AbstractBLEManagerImpl, implementing only platform-specific methods such as PlatformInit and PostDriveBLEState.
• Added the new shared source file (src/platform/silabs/AbstractBLEManagerImpl.cpp) to the relevant GN build targets for both efr32 and SiWx917.

Related issues

N/A

Testing

Manual testing was not performed due to environment limitations. Build verification with gn format/gn gen and a full build is recommended to confirm the refactor integrates cleanly.

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