build system to create flag storage files per partition

Add make file targets to create storage files. Note the container field
to aconfig command is an empty string for now as flags now by default
assumes empty container string in parsed_flag proto. Need to update it
once the container specification to aconfig files are done.

Bug: b/312239352
Test: m
Change-Id: If7bd12be5917a4779047633c00f88166574bfe0b