Create a "current_product" named platform.

This enables output paths of product agnostic targets (e.g. mainline
apex) to be identical when switching between target products, since the
product name is no longer embedded in:

1) the output path prefix in bazel-out
2) the PlatformOptions contribution to the configuration hash as part of a transition

Test: treehugger
Change-Id: I3fd0f7134209aa7eede3ed98412a1c5482689b57