Android: Fix include path for unified headers

In commit v3.9.0-rc3~3^2 (Android: Add support for unified headers,
2017-06-12) we accidentally constructed the unified header include
directories from the linking sysroot.  Construct them from the
compiling sysroot instead.

Fixes: #16584
Status Job ID Name Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check