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
diff --git a/android/config.go b/android/config.go
index b3ff86b..9f5ce5f 100644
--- a/android/config.go
+++ b/android/config.go
@@ -2084,3 +2084,7 @@
 func (c *deviceConfig) NextReleaseHideFlaggedApi() bool {
 	return Bool(c.config.productVariables.NextReleaseHideFlaggedApi)
 }
+
+func (c *deviceConfig) ReleaseExposeFlaggedApi() bool {
+	return Bool(c.config.productVariables.ReleaseExposeFlaggedApi)
+}