aconfig: add assign_flag_ids function

Add a function to assign flag id for flags in a package. We first assert
that the flags should have been sorted by its name, then assign a u32 id
to each flag in order. Note this cl adds the function, it will be used
in later cls.

Bug: b/312243116
Test: atest aconfig.test
Change-Id: I843433305abecbe0cc78988fe23dcf35c6a03358