Build aconfig storage files from soong filesystem

`gen_aconfig_flags_pb: true` additionally generates, aconfig storage
files in `etc/aconfig` directory. That includes, package.map,
flag.map and flag.val files.

Bug: 329187549
Test: m aosp_cf_system_x86_64
Change-Id: Ibbfde390cd84ca06bdd345ce0d0b4baa30a60773