RFR: 8298416: Console should be declared `sealed` [v2]

[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. The implementation resided in `Console` class is separated into `ConsoleImpl` class.

Naoto Sato has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Making fields final, other clean-ups
 - Merge branch 'master' into JDK-8298416-SealedConsole
 - tidying up
 - sealed Console, impl/if separation

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11615/files
  - new: https://git.openjdk.org/jdk/pull/11615/files/b5bc9f5e..a21798ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11615&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11615&range=00-01

  Stats: 5674 lines in 220 files changed: 2650 ins; 2237 del; 787 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