Bazel overlay: write all .bzl files in <bazel_overlay>/build/bazel/overlay_rules directory

This keeps the top level directory clean, and leverages the
new "build/bazel" repo location / package path.

Test: m bazel_overlay && cd out/soong/bazel_overlay && bazel cquery //...

Bug: 171180724

Fixes: 171180724
Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: I151b8f3d2177167711e5974e5eabad8d6e910276