RFR: 8285285: Avoid redundant allocations in WindowsPreferences

[Andrey Turbanov]

1. No need to call `String.substring` if you need need to compare start of string with some constant. `String.startWith` suites better.
2. Unused String array is allocated in `childrenNamesSpi` method

-------------

Commit messages:
 - [PATCH] Avoid redundant allocations in WindowsPreferences
 - [PATCH] Avoid redundant String allocation in WindowsPreferences.toJavaName

Changes: https://git.openjdk.java.net/jdk/pull/8322/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8322&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285285
  Stats: 20 lines in 1 file changed: 0 ins; 2 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8322.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8322/head:pull/8322

PR: https://git.openjdk.java.net/jdk/pull/8322