[android] Add libtorch headers to pytorch_android aar (#39507)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39507
Adding gradle task that will be run after `assemble` to add `headers` folder to the aar.
Headers are choosed for the first specified abi, they should be the same for all abis.
Adding headers works through temporary unpacking into gradle `$buildDir`, copying headers to it, zipping aar with headers.
Test Plan: Imported from OSS
Differential Revision: D22118009
Pulled By: IvanKobzarev
fbshipit-source-id: 52e5b1e779eb42d977c67dba79e278f1922b8483
