Revert "Revert "aconfig: @hide generated Java code""

This reverts commit c0e8ffea41306eedf099866ccd8e4c28851a0b3d.

Reason for revert: The detected build breakage was unrelated to the aconfig change. Details in b/297881670#comment11.

Change-Id: Ia32bc2e796ed3de7fa049c2c2db340f4325a2853
diff --git a/tools/aconfig/templates/Flags.java.template b/tools/aconfig/templates/Flags.java.template
index 7f04700..95a7f3b 100644
--- a/tools/aconfig/templates/Flags.java.template
+++ b/tools/aconfig/templates/Flags.java.template
@@ -1,5 +1,6 @@
 package {package_name};
 
+/** @hide */
 public final class Flags \{
 {{- for item in class_elements}}
     public static final String FLAG_{item.flag_name_constant_suffix} = "{item.device_config_flag}";