Remove --noexperimental_platform_in_output_dir

This is a followup to aosp/2606989.

This flag is not necessary now that we're using one platform name
for all of mixed builds.

Also rename current_product to mixed_builds_product so that it's clear
that that this platform should only be used for mixed builds.

In addition, make the bazelrc files point to the named products again
instead of the mixed build product so that b builds will still have
qualified outputs, but mixed builds won't.

Test: Presubmit and kernel build tools abtd run
Change-Id: I7f764cf42cd1323f4b495d1320931f59a076ac63