RFR: 8358015: Fix SequencedMap sequenced view method specifications

[Stuart Marks]

For a full explanation, see the bug report [JDK-8358015](https://bugs.openjdk.org/browse/JDK-8358015).

This PR includes three related changes:
* New overrides in SequencedMap view collection implementations, which improve their behavior.
* Update to `@implSpec` clauses to reflect this behavior (which was previously ill-specified).
* Update to a test which relied on unspecified behavior.

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Commit messages:
 - Update @implSpecs of SequencedMap view methods.
 - Update SequencedMap views to use SequencedMap methods instead of inheriting

Changes: https://git.openjdk.org/jdk/pull/25515/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25515&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358015
  Stats: 69 lines in 3 files changed: 59 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/25515.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25515/head:pull/25515

PR: https://git.openjdk.org/jdk/pull/25515