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