Revert "Revert "Expand @FlaggedApi(FLAG) constants in API signature files""

This reverts commit 157d0c189f3177016695dd26bb5f4435a8dfd628.

Reason for revert: revert with fix to java_defaults instead of filegroup

Test: m checkapi
Test: m <partition>/etc/aconfig_flags.textproto & diff against a clean
      build
Change-Id: I757c6e87d81768ef6095a4bea67c74c3ae6028a7
diff --git a/api/StubLibraries.bp b/api/StubLibraries.bp
index 2f84df7..f345f61 100644
--- a/api/StubLibraries.bp
+++ b/api/StubLibraries.bp
@@ -29,6 +29,9 @@
 
 droidstubs {
     name: "api-stubs-docs-non-updatable",
+    srcs: [
+        ":framework-minus-apex-aconfig-srcjars",
+    ],
     defaults: [
         "android-non-updatable-stubs-defaults",
         "module-classpath-stubs-defaults",