RFR: 8294626: Improve URL protocol lower casing [v3]

[Claes Redestad]

> Move a simple utility method from `URL` to the shared `sun.net.util.URLUtil` class, rename it for clarity and enhance it so that it returns the string literal if the protocol matches one of the tested variants. This helps reduce dependencies from bootstrap code on `Locale` while improving the performance and allocation pressure in certain applications.

Claes Redestad has updated the pull request incrementally with two additional commits since the last revision:

 - Cleanups
 - Revert accidental import mangling

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10506/files
  - new: https://git.openjdk.org/jdk/pull/10506/files/0cf06bb6..2cec02e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10506&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10506&range=01-02

  Stats: 8 lines in 3 files changed: 5 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10506.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10506/head:pull/10506

PR: https://git.openjdk.org/jdk/pull/10506