Revert "Revert "Remove the prebuilt bionic headers.""

This reverts commit fda647dd860c00cc3691eea10182e129099c9d0a.

This may break some unbundled branches. If you're building NDK code in
your branch you'll need to add the projects that generate the NDK
sysroot. You'll primarily need platform/bionic, but if you're using
any NDK frameworks APIs you'll need those frameworks projects as well.

Building the NDK also depends on libclang, so you'll need
external/clang and the dependencies of libclang.

Test: make checkbuild
188 files changed