Contains:
59068ee4c examples/fractal.cpp: Don't use rand()
7df53dd16 Add primes example
ded37ceb8 Update README.md with build instructions
b80c797df CMakeLists.txt: Don't default to building tests.
d89fe34b6 CMakeLists.txt: Use explicit file lists
757566df0 Update README.md
8c98371e5 Presubmits: Enable warnings-as-errors.
0025389a1 CMakeLists: Add option to treat warnings as errors
eeb070293 Fix all compiler warnings

Commands:
git subtree pull --prefix third_party/marl https://github.com/google/marl 59068ee4cf1f5ff5e691ff010c8d83b5f862c4fa --squash

Bug: b/140546382
Change-Id: Ie37d6e1ce315aaeabeee5e006c5e1f6a7a5ca184