Remove DialpadActivity

It is never used anymore. We're using Dial*tacts*Activity for Phone.

Intent with "com.android.phone.action.TOUCH_DIALER" has been used
inside the Activity and has never supported it for external use.
We'll keep the Intent for now, but we won't support correct behavior
for it. Using android.intent.action.DIAL should do what 3rd party
app will want.

TESTED:
- app works as usual with DIAL intent
- all unit tests run correctly
- intent with TOUCH_DIALER introduces Dialpad screen in
  DialtactsActivity

Bug: 5160191
Change-Id: I77d38fb3f7b51782fab3fee7b9e732f55ecf4654