Fix findbugs violations in openflowplugin-impl

Fix findbugs violations in openflowplugin-impl

- Method ignores exceptional return value
- Check for sign of bitwise operation
- Consider using Locale parameterized version of invoked method
- Reliance on default encoding
- Synchronization performed on util.concurrent instance
- Unchecked/unconfirmed cast
- Unchecked/unconfirmed cast of return value from method
- Return value of method without side effect is ignored
- Dead store to local variable
- Method uses the same code for two branches
- Load of known null value
- Parameter must be non-null but is marked as nullable
- Field not initialized in constructor but dereferenced without null check

Change-Id: I92e00a5772038111d1ab8d9f53d52c43e3f4909d
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>