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

android/variable.go

@@ -498,6 +498,8 @@ type ProductVariables struct {
 	BuildIgnoreApexContributionContents []string `json:",omitempty"`
 
 	HiddenapiExportableStubs *bool `json:",omitempty"`
+
+	ExportRuntimeApis *bool `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {