AAPT2: Introduce notion of 'product' to ResourceTable

This allows us to preserve the various product definitions during the compile
phase, and allows us to select the product in the link phase.

This allows compiled files to remain product-independent, so that they do not need
to be recompiled when switching targets.

Bug:25958912
Change-Id: Iaa7eed25c834b67a39cdc9be43613e8b5ab6cdd7