Move vendor and product variant generation logic from cc package to android package

Although image variation generation logic has moved out of cc package to
the android package, the vendor and product partition variants
generation logic is still specific to cc package. Therefore, in order to
create a product or vendor variant, they have to specified in
`ExtraImageVariants`. In order to avoid such confusing behaviors and
enforce modules to specify product and vendor installation rules, this
change moves the vendor and product variant generation logic to
android.ImageInterface.

Test: m nothing --no-skip-soong-tests && diff contents of out/soong/Android-{product}.mk
Change-Id: I9e14f3739d9dea94167ee6a91e92b2f942055aba