server: refactor valid_signal
server: refactor valid_signal

Change the method name to bus_normalize to more accurately reflect what
it does, which is normalize unique and well-known bus names to a
well-known name.

Provide the normalized name to callers.
Fix-up call sites with the new name.

Tested: Manually verified mapper content is the same for a test object
  after adding and removing interfaces from the test object, after new
  service discovery and after bus name owner loss.
Change-Id: I4bc194b3d5537eadf3e6b0a874fe72caaf615372
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-objmgr,openbmc/phosphor-objmgr