b/12068020 Make kb layouts only unique to vendor/product

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: Id0890d13e1c859eaf993d4831b7b1acbaf5df80f