aconfig_storage_file: add flag_info definition

Flag info file is another memory mapped file to store the file
information such as if current flag has been staged or not, is it sticky
and etc. This file will be created by aconfig storage daemon during
container initialization.

Bug: b/321077378
Test: atest aconfig_storage_file.test
Change-Id: I8f70e04c74b5770f89dbe981d9f03c787ea5eeed