BUILD, WORKSPACE.

Also check in base skeletal ninja_build and ninja_graph targets.

Bug: 170609618
Fixes: 170609618
Test: (setup WORKSPACE file) . build/envsetup.sh && croot && ln -s build/bazel/bazel.WORKSPACE WORKSPACE
Test: linux with aosp/1457075 -> ./prebuilts/bazel/linux-x86_64/bazel_nojdk-3.6.0-linux-x86_64 --bazelrc=build/bazel/linux.bazelrc info java-home
Test: darwin with aosp/1456077 -> ./prebuilts/bazel/darwin-x86_64/bazel_nojdk-3.6.0-darwin-x86_64 --bazelrc=build/bazel/darwin.bazelrc info java-home

Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: Idc4cd6bc8ff3415764df8561bd8503f4c0dde44e