RFR: 8336654: [lworld] Tests depending on sun.awt.AppContext can fail when run with migrated classes

[Phil Race]

The problem is that Boolean will be a value type in the future (Project Valhalla)
and so it can't be the referent of a Reference.

In this code, the abstract class that creates the Reference accepts a generic type so isn't in control of what it receives.

The concrete class that extends the generic class has no idea what the super class implementation does,
so has no way to know that it might do something that is invalid for  a value type.

That's an interesting observation for Valhalla (that the code that does something inappropriate for a value type
isn't the source of the value type) but I don't think we need to be concerned
about that here because we can see reasons to do this anyway.

The specific classes exist because a nominal singleton for the VM may need to be private to an AppContext.
This is  a concept from applets but also from webstart where we have a single JVM that may be running code
from different origins that needs to be partitioned and sand boxed.

This concept is obsolete now as applets and webstart are no longer supported, and the Security Manager
is disabled, further undermining any way to support partitioning.

This specific case of a Boolean created from the value of a Java System Property looks like it never needed
this partitioning, so the minimal fix is to stop using RecyclableSingleton for it, and just cache the value of that.

The next  level of fix is to note that since AppContext is obsolete, there is no need for RecyclableSingleton
to use AppContext specific values, making RecyclableSingleton just a lazy initialization mechanism for the singleton.

Given that removing the obsolete AppContext is on the TBD list - and some JDK components have already
removed per-AppContext code - and we'd probably come back to this in JDK 25 anyway it seems best
to get it over with.

That does mean that one other place in sun.awt.ImageCache needed to be updated too.

Also one test that explicitly checked that AppContexts were used for UI instances is obsolete and removed.

Note that RecyclableSingleton also offers some lazy initialisation benefit, and is widely used in Aqua,
so going further and removing that probably isn't something we want to do at all, and certainly not here.

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Commit messages:
 - 8336654

Changes: https://git.openjdk.org/jdk/pull/22868/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336654
  Stats: 146 lines in 4 files changed: 7 ins; 134 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22868.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22868/head:pull/22868

PR: https://git.openjdk.org/jdk/pull/22868