GH-50637: [C++] Register bundled Abseil libraries in build_absl()

[Reranko05]

What changes are included?

Move the registration of bundled Abseil static libraries from build_protobuf() to build_absl().

Since the Abseil targets are defined in build_absl(), registering them there keeps the bundled dependency registration with the component that provides them instead of coupling it to the Protobuf build logic.

Are these changes tested?

• Configured Arrow successfully with CMake.
• Built Arrow successfully with the default configuration.
• CI will verify the change across supported platforms, including the Windows configuration where this issue was originally observed.

Are there any user-facing changes?

No. This is an internal CMake/build system change.

[Reranko05]

All triggered CI checks are green. If someone could trigger Windows JNI workflow, it would help verify this fix against the original issue.

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[Reranko05]

Thanks @tadeja