Implement initial ambient capabilities support.

Credit to Brian McGillion for the initial implementation (in
https://android-review.googlesource.com/#/c/302756/).

Current support allows callers to also set ambient capabilities when
using regular capabilities. A follow-up CL will clean up the preloading
situation wrt ambient capabilities.

Bug: 32066154
Test: Use 'drop_privs' executable, check that it gets ambient caps.
Change-Id: If493fb5886fe9798436a749b7ebdbc04f00000b6
5 files changed