Fix issue #11374840: adb shell am startservice not working any more

The startService() and stopServie() calls had a redundant check for
the incoming user ID being valid, but with its own custom implementation
that doesn't match the normal handleIncomingUser flow.  In fact, for
both of these we are going to do handleIncomingUser anyway when we get
to retrieveServiceLocked(), so there was just no need for this.

Change-Id: I14409a03781a14a5f1a786aceb31dcc77efb062c