<i18n dev> RFR: 8302813: awt.image.incrementaldraw can use Boolean.parseBoolean() to parse the system property

[SWinxy]

Please review this change which moves the parsing of the `awt.image.incrementaldraw` property from the static initializer block into the field itself by invoking `Boolean.parseBoolean()` on the system property getter.

Hopefully in the near future we can do away with `AccessController`s and simply go with `System.getProperty` :)

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Commit messages:
 - Make isInc private
 - awt.image.incrementaldraw can use Boolean.parseBoolean() to parse the system property

Changes: https://git.openjdk.org/jdk/pull/12639/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12639&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302813
  Stats: 7 lines in 1 file changed: 1 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12639.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12639/head:pull/12639

PR: https://git.openjdk.org/jdk/pull/12639