Rename the poorly named ALL_PRODUCTS variable.

There was PRODUCTS, ALL_PRODUCTS and really_all_products, and
ALL_PRODUCTS was in fact not all products. Rename it to what
it really is: parent makefiles, i.e. product makefiles that include
other product makefiles.

Also rename the really_all_products var.

Bug: 80410283
Test: lunch aosp_arm64; m product-graph
Change-Id: I87aec49966e50c364ca2baf5de10fe30f9292a53