Fix findbugs violations in openflowplugin

Fix findbugs violations in openflowplugin

- Comparison of String parameter using == or !=
- Comparator doesn't implement Serializable
- Field is a mutable collection
- Incorrect lazy initialization of static field
- Inefficient use of keySet iterator instead of entrySet iterator
- Private method is never called
- Unsigned right shift cast to short/byte
- Class is final but declares protected field
- Consider returning a zero length array rather than null

Change-Id: I714bd4005a33a772b0b76b0c0f89f28b8517cd06
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>