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/config.go

@@ -2084,3 +2084,7 @@ func (c *deviceConfig) CheckVendorSeappViolations() bool {
 func (c *deviceConfig) NextReleaseHideFlaggedApi() bool {
 	return Bool(c.config.productVariables.NextReleaseHideFlaggedApi)
 }
+
+func (c *deviceConfig) ReleaseExposeFlaggedApi() bool {
+	return Bool(c.config.productVariables.ReleaseExposeFlaggedApi)
+}