aconfig: @hide generated Java code

Flagged APIs are annotated using `@FlaggedApi(Flags.FLAG_NAME)`, where
Flags.FLAG_NAME was auto-generated by aconfig.

When metalava generates an API signature file, it can either

  a) insert the value of the constant ("com.foo.bar.flag_name"), or
  b) insert the name of the constant (FLAG.NAME)

In the case of @FlaggedApi, we want a). This requires that

  x) metlava has access to the definition of the constant while generating the API signature file, and
  y) the constant is not part of the API surface of the API signature file

x) is handled by the build system, y) is handled by the aconfig code
generation.

This CL @hide:s all generated Java code, to make sure it is accessible
within the platform, but never part of any API surface.

Bug: 297881670
Test: atest aconfig.test aconfig.test.java
Change-Id: I328ed1a652a4e5e293f2f4b11f916d29fc2fbcbd