RFR: 8300868: Reduce visibility in java.io.SerialCallbackContext

[Per Minborg]

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

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Commit messages:
 - Reduce visibility in j.i.SerialCallbackConext

Changes: https://git.openjdk.org/jdk/pull/12143/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300868
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 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