RFR: 8325858: Replace Unsafe usage in XEmbeddingContainer with FFM API

[Per Minborg]

This PR proposes to remove the use of `Unsafe` in the class `XEmbeddingContainer ` and replace it with the supported FFM API.

I tried to make this PR as small as possible while opening up for migration of other classes later on (such as `XEmbedServer` which can be modified similarly under a separate PR).

There are also two drive-by fixes in this PR:
 * Moved JavaDocs for `XAtom` to its proper location and fixed two typos in the text
 * Declared a field in `XEmbeddingContainer` as `privat final`

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Commit messages:
 - Replace Unsafe usage in XEmbeddingContainer with FFM API

Changes: https://git.openjdk.org/jdk/pull/17846/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325858
  Stats: 80 lines in 3 files changed: 49 ins; 11 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/17846.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17846/head:pull/17846

PR: https://git.openjdk.org/jdk/pull/17846