Use all *.aconfig source in flags BP instead of maintaining a list

Adding sources one by one isn't necessary, and causes merge conflicts
each time a new one is added (assuming you've added one too).

We can help everyone skip that step and use all sources in the flags
folder automatically.

Tag: #refactor
Bug: 317394189
Flag: EXEMPT build file change only
Test: m com.android.btservices
Change-Id: I506b1f9453a6cab7db2bcd7263ad331829c0d2a1