aconfig: add @UnsupportedAppUsage to methods in generated flags

The java generated flags are marked as @hide which breaks CTS. To
unblock CTS test add @UnsupportedAppUsage to expose the methods.

Test: atest aconfig.test.flags
Bug: 301272559
Ignore-AOSP-First: Need to commit with changes in internal main. This
change will be cherry picked to AOSP once merged in main
Change-Id: I897573c054e70fc6e093869ba6d416c38f81e28f