NDK library: extract ABI representations with STG

Replace the abidw->stg pipeline by just stg for extraction directly from
ELF/DWARF. This is to migrate to STG entirely.

As a migration tooling, keep the existing method active via
`legacy_use_abidw` as a flag on ndk_library.

Bug: 156513478
Change-Id: Ideaa9908b31591f49f9a167cfa3f3d5c95d8b198
Signed-off-by: Matthias Maennich <maennich@google.com>