RFR: 8300868: Reduce visibility in java.io.SerialCallbackContext [v2]

[Per Minborg]

> This PR proposes to reduce the visibility for constructor and methods in `java.io.SerialCallbackContext`.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright year

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12143/files
  - new: https://git.openjdk.org/jdk/pull/12143/files/6136a891..7f807539

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12143&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12143&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12143.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12143/head:pull/12143

PR: https://git.openjdk.org/jdk/pull/12143