aconfig: explicitly @hide FLAG_ constants

Because of a bug in metalava [1], the flag constants are incorrectly
considered not-hidden when metalava generates the API signature files.
This means the FLAG_ constant name will be used, instead of the FLAG_
constant value, which is what we want.

While waiting for b/300211291 to be fixed, explicitly mark the FLAG_
constants as hidden (and keep marking the classes as such).

This CL can be reverted when b/300211291 is fixed.

1. Item.isHiddenOrRemoved does not check if its parent item is hidden,
   tracked in b/300211291

Bug: 297881670
Test: atest aconfig.test aconfig.test.java
Change-Id: I05de8344ccb424bee51c39f9609489ad312397ce

tools/aconfig/src/codegen_java.rs

@@ -135,10 +135,15 @@ mod tests {
     package com.android.aconfig.test;
     /** @hide */
     public final class Flags {
+        /** @hide */
         public static final String FLAG_DISABLED_RO = "com.android.aconfig.test.disabled_ro";
+        /** @hide */
         public static final String FLAG_DISABLED_RW = "com.android.aconfig.test.disabled_rw";
+        /** @hide */
         public static final String FLAG_ENABLED_FIXED_RO = "com.android.aconfig.test.enabled_fixed_ro";
+        /** @hide */
         public static final String FLAG_ENABLED_RO = "com.android.aconfig.test.enabled_ro";
+        /** @hide */
         public static final String FLAG_ENABLED_RW = "com.android.aconfig.test.enabled_rw";
 
         @com.android.aconfig.annotations.AssumeFalseForR8

tools/aconfig/templates/Flags.java.template

@@ -3,6 +3,7 @@ package {package_name};
 /** @hide */
 public final class Flags \{
 {{- for item in class_elements}}
+    /** @hide */
     public static final String FLAG_{item.flag_name_constant_suffix} = "{item.device_config_flag}";
 {{- endfor }}
 {{ for item in class_elements}}