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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:64ad75f50ae96f9a8b57a9e8f88ae9a53d3c1044)
Merged-In: I897573c054e70fc6e093869ba6d416c38f81e28f
Change-Id: I897573c054e70fc6e093869ba6d416c38f81e28f