Add libcxxabi_headers filegroup am: a07162d9ed

Original change: https://android-review.googlesource.com/c/platform/external/libcxxabi/+/2853447

Change-Id: I68294603918ad45ddeb18bf9ccbf1e2280bb4423
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>