Handle product_variable asflag for cc_object.

cc_object crtbrand sets product_variable.platform_sdk_version.asflag
and will not compile correctly within mixed builds without it.

Only handles product_variables that expand product variables.

Bug: 181794963
Test: ~/aosp/build/bazel/scripts/milestone-2/demo.sh full
Change-Id: I293fcb18032aa51f63bb7b3de94abd6d1ec38180