Export build flags to soong.

This exports a map of build flags used in this release config to Soong.

Bug: 302514918
Test: manual
Change-Id: Id95c03e14cfbb91c0cb9c8c37a065b8387733317

android/variable.go

@@ -492,6 +492,8 @@ type ProductVariables struct {
 	NextReleaseHideFlaggedApi *bool `json:",omitempty"`
 
 	Release_expose_flagged_api *bool `json:",omitempty"`
+
+	BuildFlags map[string]string `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {