Move global hidden API flags generation to platform_bootclasspath

The global hidden API flags generation has moved from the singleton to
the platform_bootclasspath module type so this change provides the
paths to the additional hidden API flag configuration as part of the
definition of the platform-bootclasspath module.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Id82b45d2abd6ca2efcfd730689b1eda92d55978e
diff --git a/api/Android.bp b/api/Android.bp
index 485255f..6e83c08 100644
--- a/api/Android.bp
+++ b/api/Android.bp
@@ -308,6 +308,9 @@
 
 genrule {
     name: "combined-removed-dex",
+    visibility: [
+        "//frameworks/base/boot",
+    ],
     srcs: [
         ":frameworks-base-api-removed.txt",
         ":frameworks-base-api-system-removed.txt",