RFR: 8325858: Replace Unsafe usage in XEmbeddingContainer with FFM API [v2]

[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 three 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 `private final`
>  * In `XAtom`, use a utility method `assertAtomInitialized()` instead of the current duplicated code

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

  Improve getAtomData to use the FFM API and simplify assertions

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17846/files
  - new: https://git.openjdk.org/jdk/pull/17846/files/c9cf797c..530f0a70

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17846&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17846&range=00-01

  Stats: 81 lines in 2 files changed: 26 ins; 34 del; 21 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