Refactor core C++ files toward Google C++ style

[RogerTeng]

Motivation

• Align project-owned C++ sources with Google C++ style for consistent formatting and naming.
• Replace legacy PImpl identifiers and inconsistent constant names to improve readability and maintainability.
• Harden the sample application by adding argument validation and safer buffer handling.

Description

• Add an include guard and reformat DTLN_AEC.h, rename the private implementation type to Impl and the member to impl_, and update public API signatures accordingly.
• Rework DTLN_AEC.cpp to use consistent constant names (kWindowSize, kWindowShift, kFftForTensorSize, kNumModels, kNumThreads), apply Google-style spacing and indentation, and bracket third-party includes with // NOLINTBEGIN/// NOLINTEND.
• Refactor AecSample.cpp to validate argc, check file opens and initialization failures, return non-zero on error, and replace manual new[]/delete[] buffers with std::vector<short>.
• Apply clang-format with the Google style to the modified files to enforce the changes.

Testing

• Formatted files in-place with clang-format -style=Google -i DTLN_AEC/DTLN_AEC.h DTLN_AEC/DTLN_AEC.cpp AecSample.cpp and the command completed successfully.
• Verified formatting with clang-format --dry-run --Werror -style=Google AecSample.cpp DTLN_AEC/DTLN_AEC.cpp DTLN_AEC/DTLN_AEC.h and the check passed.
• A dry-run without an explicit Google style using clang-format --dry-run --Werror reported formatting differences and failed as expected.

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Codex Task