NDK library: switch to stg ABI format

STG's native .stg format has been designed with version control in mind
as is the best format to represent STG's internal data representation.

Hence, migrate the ABI representation dump to .stg. That is done by
converting `abidw` extracted xml to stg at build time. STG has builtin
`abitidy` functionality, so `stg` can replace the prior `abitidy` step
to generate the final format.

Bug: 156513478
Change-Id: Ifcac8998ab899637a5d8c54f9e72d79d3e28ec52
Signed-off-by: Matthias Maennich <maennich@google.com>