aconfig: add a new aconfig storage file flag listing api

added a new function called list_flag_with_info to list all the flags
given all four storage files (package.map, flag.map, flag.val, flag.info).
also exported this api thru cxx interface so aconfigd can use it.

Bug: b/312444587
Test: atest -c
Change-Id: Ibbfe657b980d40e25e5e28962b930338192e2d98