Convert the build flag RELEASE_EXPORT_RUNTIME_APIS to a product variable

RELEASE_EXPORT_RUNTIME_APIS build flag is currently used to modify the
filter condition for the "exportable" stubs, so that it exposes wider
set of flagged apis (ENABLED && READ_ONLY vs ENABLED || READ_WRITE).
This flag was originally introduced as a build flag to be enabled in
specific release configurations, but instead this is used as a product
variable that will be enabled in the product `sdk_with_runtime_apis`,
which will be introduced in a future change.

Test: m nothing --no-skip-soong-tests
Bug: 326312949
Change-Id: Ieb3cee517dd6cedcca844a9eab270aedf8199d4e

java/droidstubs_test.go

@@ -22,6 +22,8 @@ import (
 	"testing"
 
 	"android/soong/android"
+
+	"github.com/google/blueprint/proptools"
 )
 
 func TestDroidstubs(t *testing.T) {
@@ -419,8 +421,8 @@ func TestReleaseExportRuntimeApis(t *testing.T) {
 		android.FixtureModifyProductVariables(func(variables android.FixtureProductVariables) {
 			variables.BuildFlags = map[string]string{
 				"RELEASE_HIDDEN_API_EXPORTABLE_STUBS": "true",
-				"RELEASE_EXPORT_RUNTIME_APIS":         "true",
 			}
+			variables.ExportRuntimeApis = proptools.BoolPtr(true)
 		}),
 		android.FixtureMergeMockFs(map[string][]byte{
 			"a/A.java":      nil,