Fix "make product-graph" and "make dump-products".

- For unmodified "make product-graph" and "make dump-products",
  load only the current product configuration makefiles. This is much
  faster than loading all product makefiles.
- For "make product-graph ANDROID_PRODUCT_GRAPH=--all",
  "make dump-products ANDROID_DUMP_PRODUCTS=all", load all product
  makefiles.
- Move product-graph.mk out of build tasks, so we can skip loading all
  the Android.mks, which takes long and we don't really need them.
  More importantly, with all product makefiles loaded, modules in
  Android.mks are prone to clash (if they are conditionally included
  by variables set up in product makefiles) and lead to parse-time
  error.

Change-Id: Idc1d6b0c23eb2c8bb34fdd7a1fa4d56171768d21