Clean up Cronet native logging level code

- Make it clear this sets the *native* Chromium code log level, not the
  Android or Java log level
- Native Chromium LOG() statements are logged with the "chromium"
  Android log tag, so use that tag to enable VLOG() instead of using the
  CronetUrlRequestContext log tag (which is incredibly confusing)
- Move the code to CronetLibraryLoader - this is Cronet library init
  code and has no business living in CronetUrlRequestContext (also,
  that's racy).
- Don't send the reader on a Fedex quest fetching random constants at
  the other end of the source file
- Add a TODO to revisit the whole approach as it's basically a hack

Change-Id: I52d150e87c9042ac86bfede356d800540bcedf05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4907020
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Auto-Submit: Etienne Dechamps <edechamps@google.com>
Commit-Queue: Stefano Duo <stefanoduo@google.com>
Cr-Commit-Position: refs/heads/main@{#1204056}