Dialer: Merge ConstantsImpl into Constants [18/x]

* We don't have more than one Implementation, so no need to use
  reflection to find our actual Constants - so merge them into one
* Remove getSettingsActivity() while at it, it's unused
* Remove getUserAgent() as well, we only return null which we can use
  to simplify the call to it

Change-Id: I1f2cdf4842d161e431b28ea8aa4d2230a8d06739