RFR: 8278860: Streamline properties for Monocle

[Johan Vos]

Base decisions in prism for embedded cases on the same glass.platform property that is also used in glass.
This PR replaces the property `embedded` with the property `glass.platform` .

There is only 1 place where the property `embedded` was read, which is in the PlatformUtil.java.

There are only 2 places where the value of that property is used, and in both cases the only check is to detect whether or not that property has a value of "monocle".

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Commit messages:
 - Base decisions in prism for embedded cases on the same glass.platform property

Changes: https://git.openjdk.java.net/jfx/pull/696/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=696&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278860
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/696.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/696/head:pull/696

PR: https://git.openjdk.java.net/jfx/pull/696