[jdk11u-dev] RFR: 8218917: KeyEvent.getModifiers() returns inconsistent values for ALT keys

[Amos Shi]

Backport of [JDK-8218917](https://bugs.openjdk.org/browse/JDK-8218917)

Testing
- Local: Test passed
  - `AltGraphModifierTest.java`: Test results: no tests selected
  - `AltKeyBug.java`: Test results: passed: 1
- Pipeline: 
- Testing Machine:

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Commit messages:
 - Backport 2044245c90810075fbead432ad92de8fcb8af68a

Changes: https://git.openjdk.org/jdk11u-dev/pull/2587/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8218917
  Stats: 123 lines in 3 files changed: 108 ins; 11 del; 4 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2587.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2587/head:pull/2587

PR: https://git.openjdk.org/jdk11u-dev/pull/2587