aconfig: modify_parsed_flags_based_on_mode return Result

This commit changes modify_parsed_flags_based_on_mode to return Result
instead of vector. This change will have two consequences.
1. Error will be thrown if no flags fit into the current codegen mode
2. Error will be thrown if the flag declaration is empty

Bug: 311152507
Test: atest aconfig.test
Change-Id: I69b9a84312faed9f757bf3974b3cea49c5c5e285