b/12068020 Make kb layouts only unique to vendor/product. Do not merge

This is a cherry-pick of https://googleplex-android-review.git.corp.google.com/#/c/399886/

Instead of storing a kb layout per device descriptor (which is expected
to be unique), store it for each vendor/product. This way we can keep
a consistent layout between identical but physically different keyboards.

There are some corner cases this is expected to fail on, namely devices
that incorrectly have the same vendor/product id. Devices that don't
define a vendor/product id will continue to use the descriptor to store
layout files.

Change-Id: I1f2508561992080459310d5a644dad65a9c24f1a