Disallow overriding services that were set

Disallow overriding services that were set

Otherwise setService() calls (including indirect, like via
setContentLang() ) will be silently overridden.  This bit me several
times already while working on tests.  If someone actually wants to do
this, they should probably split their test in two, because it evidently
has two separate phases that they're testing (with/without services
set).

Depends-On: I9acb81c0de95eb5a6bed543d757ae62523ea6041
Change-Id: I8c60e37c179320e61684cbc11281c509e525e8fb