Reduce logging in AconfigFlags.java

By default, the only logging that should be generated is when an element
is skipped due to an disabled or enabled feature flag. The more verbose
logging needs to be enabled by setting `DEBUG = true`.

Also cleaned up some unused code and refactored constants.

Bug: 369907949
Test: Full build
Flag: EXEMPT flagging support change
Change-Id: Ie61bff7e39fa3136e95f773afb359a03c90eef1a