Fix findbugs violations in md-sal - part 1

Fix findbugs violations in md-sal - part 1

- sal-common-util
- sal-common-impl
- sal-dom-api
- sal-dom-spi
- sal-binding-api
- sal-inmemory-datastore

Violations:

- Non-transient non-serializable instance field in serializable class
- Field isn't final but should be
- Unchecked/unconfirmed cast
- Class names shouldn't shadow simple name of implemented interface
- Redundant nullcheck of value known to be non-null
- Field not initialized in constructor but dereferenced without null check
- Equals checks for incompatible operand
- Method ignores return value

Change-Id: I57ceba7dae12114eba962c01aea259004f4a2983
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>