RFR: 8345249: Apply some conservative cleanups in FileURLConnection [v2]

[Eirik Bjørsnøs]

> Please review this PR which applies various cleanups to `sun.net.www.protocol.file.FileURLConnection`.
> 
> This class is known to be an _old, intricate, and hard to maintain piece of code._  However, there are some relatively straightforward refactorings / cleanups possible which improve readability and makes it easier to reason about what's going on in this class.
> 
> In this PR, I have chosen to make each individual small change a separate commit. This to assist review of each individual change, which can otherwise disappear a bit when reviewing the PR as a whole. 
> 
> A detailed listing of each commit follows in a separate comment.
> 
> This is a pure cleanup PR, no tests are added or updated in this PR.

Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:

  Replace String::valueOf with Long::toString

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/22459/files
  - new: https://git.openjdk.org/jdk/pull/22459/files/f9efa3b8..16ee08c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22459&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22459&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22459.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22459/head:pull/22459

PR: https://git.openjdk.org/jdk/pull/22459