aconfig: consolidate how fully qualified flag name is calculated

Add an extension trait to consolidate how the fully qualified package
name (<package>.<name>) is calculated and use this where possible.

This CL is a semantic change only. The intention of this CL is to reduce
the risk of future bugs.

Bug: N/A
Test: atest aconfig.test
Change-Id: Ibab6641dda3843337fbea02631c31a1fdd8fb4ab