Export default RBE_platform

Export it so that we have one source of truth,
which can be used by the called tools.

Test: Use the exported value in ART build.
Change-Id: Iaa45b1780ffae11e09790e193ca2bd01e2b732a8

ui/build/Android.bp

@@ -34,15 +34,16 @@ bootstrap_go_package {
     deps: [
         "blueprint",
         "blueprint-bootstrap",
+        "blueprint-microfactory",
+        "soong-finder",
+        "soong-remoteexec",
+        "soong-shared",
         "soong-ui-build-paths",
         "soong-ui-logger",
         "soong-ui-metrics",
         "soong-ui-status",
         "soong-ui-terminal",
         "soong-ui-tracer",
-        "soong-shared",
-        "soong-finder",
-        "blueprint-microfactory",
     ],
     srcs: [
         "bazel.go",