RFR: 8298416: Console should be declared `sealed`

[Naoto Sato]

`Console` class now has a couple of internal subclasses within `java.io` package. It should be `sealed` and subclasses be declared in the `permits` clause.

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Commit messages:
 - tidying up
 - sealed Console, impl/if separation

Changes: https://git.openjdk.org/jdk/pull/11615/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11615&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298416
  Stats: 625 lines in 2 files changed: 366 ins; 237 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/11615.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11615/head:pull/11615

PR: https://git.openjdk.org/jdk/pull/11615