Expand @FlaggedApi(FLAG) constants in API signature files

The auto-generated Flags.FLAG_NAME constants are difficult to review in
API tracking files. metalava will expand annotation arguments if

  (1) the field declaration is known to metalava, and
  (2) the constant is not part of the API surface.

The auto-generated constants are hidden, so not part of any API surface.
This satisfies (1).

This CL adds the auto-generated sources to metalava's input. This
satisfies (2).

(cherry picked from commit 01544b94)

Bug: 297881670
Test: m checkapi
Test: m <partition>/etc/aconfig_flags.textproto & diff against a clean build
Merged-In: I757c6e87d81768ef6095a4bea67c74c3ae6028a7
Change-Id: I757c6e87d81768ef6095a4bea67c74c3ae6028a7