DERBY-5840: Clean up compiler warnings introduced by using Java 5 language features

- Make ArrayUtil.asReadOnlyList() use checked conversions and remove
  SuppressWarnings annotation

- Generify the signature of ArrayUtil.copy(Object[]), remove the now
  redundant copy(String[]) overload, and remove the now redundant
  casts in many of the call sites

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1485676 13f79535-47bb-0310-9956-ffa450edef68