soong: -Werror=fortify-source

Recently, we started deferring to clang for some of its built-in
FORTIFY-like warnings. We should always treat these as errors, since
they represent potential security vulnerabilities.

Bug: 131861088
Test: m. Also made a compilation in Bionic fail; verified the build
  command had -Werror=fortify-source in it.

Change-Id: I2715ea411ef067f801534dab4d306fef5431f290