Fix config file fetching

1. Make config file fetching run in background again if a file already
   exists - this behavior was changed in
   https://android-review.git.corp.google.com/c/platform/build/soong/+/2059887
   by accident.
2. Always try to load a config file even if fetching a config file fails
   - this behavior was changed in
     https://android-review.git.corp.google.com/c/platform/build/soong/+/2380792
     iiuc.

Bug: b/207551843
Test: Ran builds with no config file present, with config file present
and confirmed that config file fetching happens in the background now.

Change-Id: I6321684a0e17df3371f3f415602e7fa33cfba454