Move libimapper_stablec ABI dumps from prebuilts/abi-dumps

The update process for prebuilts/abi-dumps/platform will be adapted for
trunk stable. The ABI dumps for libimapper_stablec are moved to
"abi-dumps" in this project so that the trunk stable release process
will not affect them.

Test: m libimapper_stablec_abicheck
Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py \
      -libs libimapper_stablec_abicheck --lib-variant PLATFORM \
      -ref-dump-dir hardware/interfaces/graphics/mapper/stable-c/abi-dumps \
      -products aosp_arm64
Bug: 333532038
Change-Id: Ia69daa721ef983c18118bd6d0a70dd5b7a0a47a2