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
diff --git a/bazel/properties.go b/bazel/properties.go
index abdc107..38e32e7 100644
--- a/bazel/properties.go
+++ b/bazel/properties.go
@@ -16,6 +16,7 @@
 
 import (
 	"fmt"
+	"regexp"
 	"sort"
 )
 
@@ -31,6 +32,8 @@
 
 const BazelTargetModuleNamePrefix = "__bp2build__"
 
+var productVariableSubstitutionPattern = regexp.MustCompile("%(d|s)")
+
 // Label is used to represent a Bazel compatible Label. Also stores the original bp text to support
 // string replacement.
 type Label struct {
@@ -144,3 +147,23 @@
 		panic(fmt.Errorf("Unknown arch: %s", arch))
 	}
 }
+
+// TryVariableSubstitution, replace string substitution formatting within each string in slice with
+// Starlark string.format compatible tag for productVariable.
+func TryVariableSubstitutions(slice []string, productVariable string) ([]string, bool) {
+	ret := make([]string, 0, len(slice))
+	changesMade := false
+	for _, s := range slice {
+		newS, changed := TryVariableSubstitution(s, productVariable)
+		ret = append(ret, newS)
+		changesMade = changesMade || changed
+	}
+	return ret, changesMade
+}
+
+// TryVariableSubstitution, replace string substitution formatting within s with Starlark
+// string.format compatible tag for productVariable.
+func TryVariableSubstitution(s string, productVariable string) (string, bool) {
+	sub := productVariableSubstitutionPattern.ReplaceAllString(s, "{"+productVariable+"}")
+	return sub, s != sub
+}