Copy ABI dumps from current/64 to FINAL_PLATFORM_SDK_VERSION/64

Because prebuilts/abi-dumps/ndk/current contains additional ABI dumps
that are not included in the finalization process, the script copies
only the subdirectory to the finalized ABI dump directory.

Test: ./finalize-sdk-rel.sh
Bug: 276503801
Change-Id: I76287cf110323baf40035a957564a1e8f39e8015