Generate product build props always

GSI do not build product partition, and contains the product
contents under /system/product/. Original logic do not generate
the product build props on this case.

The patch fixes the logic for product build props generation to
align the other partitions.

Bug: 157191627
Test: build aosp_arm64-userdebug
Test: check /system/product/build.prop in out folder
Change-Id: I350bbf8c024f747c86c6c54065d690f6b46f30c6
Merged-In: Ia88c8efecb18db9c27baa466e492bfd898b581d3