release_config: cleanup how default values are used

Set the flag value to the default value, rather than waiting until the
end to check if it was set anywhere.

This matters when the flag is declared `appends=True`.

Bug: none
Test: manual
Ignore-AOSP-First: Will CP, testing on internal first.
Change-Id: I8384cf8e0e0caedb5fb5a343f8be23f37bf4dc87
Merged-In: I8384cf8e0e0caedb5fb5a343f8be23f37bf4dc87