Move the android_platform next to it's entrypoint product config file

Because we're going to start generating partition images for the
product, and the partitions will eventually be checked in, we want
them to be in sensible locations. And the platform should be there as
well so all the targets for a product are co-located.

Bug: 297269187
Test: m nothing && b build --config=android //build/bazel/examples/apex/minimal:build.bazel.examples.apex.minimal
Change-Id: Iaa25c44aa00295ada279d5fd49b5498bbafb89d5