RFR: 8325858: Replace Unsafe usage in XEmbeddingContainer with FFM API [v3]

[Per Minborg]

On Wed, 14 Feb 2024 18:09:15 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> 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
>> 
>> Tested and passed tier1-5
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Suppress restricted warning

I ran an analysis tool that looked at about one million libraries (JARs) and there was just on occurrence of any of the three classes in user code: `de.mirkosertic.bytecoder:java.desktop:jar:2023-05-19`

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PR Comment: https://git.openjdk.org/jdk/pull/17846#issuecomment-1959032622