Add a new platform_bootclasspath module type

Initially, this is just a placeholder but functionality will be added
in follow up changes.

Bug: 177892522
Test: m nothing
Change-Id: I890b0d5a117c51a19c9ac5df98c766761d3aa16c

java/Android.bp

@@ -50,6 +50,7 @@ bootstrap_go_package {
         "kotlin.go",
         "lint.go",
         "legacy_core_platform_api_usage.go",
+        "platform_bootclasspath.go",
         "platform_compat_config.go",
         "plugin.go",
         "prebuilt_apis.go",
@@ -79,6 +80,7 @@ bootstrap_go_package {
         "java_test.go",
         "jdeps_test.go",
         "kotlin_test.go",
+        "platform_bootclasspath_test.go",
         "platform_compat_config_test.go",
         "plugin_test.go",
         "rro_test.go",