Add soong config variable Release_expose_flagged_api

The variable is a release config variable which will be used to
determine whether if the api marked @FlaggedApi is exposed or not.

Test: m nothing
Bug: 299570421
Change-Id: I5647608065543cf5059836f6d6e8906a23145541

android/variable.go

@@ -490,6 +490,8 @@ type ProductVariables struct {
 	PartitionVarsForBazelMigrationOnlyDoNotUse PartitionVariables `json:",omitempty"`
 
 	NextReleaseHideFlaggedApi *bool `json:",omitempty"`
+
+	ReleaseExposeFlaggedApi *bool `json:",omitempty"`
 }
 
 type PartitionVariables struct {