StubObject: Use 'self' instead of 'StubObject' (or <ClassName>)

StubObject: Use 'self' instead of 'StubObject' (or <ClassName>)

'self' is only 4 chars (shorter to type) and reduces risk (if class name
is changed) which is very unlikely to break things. PHP replaces self with
the actual class name. Plus we're already using this in our code base.

Change-Id: Ia565ab2e4258d0ac201b14b712d1bc3bfe4e4d94