First pass for Dialer onboarding flow

* Add OnboardingActivity that controls the onboarding the UI
ensuring that the user grants the necessary permissions before
the Dialer can start.
* Add first pass (no graphics, eyeballed measurements) for the
screens that request for default dialer as well as permissions
* OnboardingActivity is not actually launched at this moment - will
be tied in to the various Dialer activities in a follow up CL.
* Add tests for logic that controls the display of the screens in
anticipation of future additions to the onboarding flow.
* Add mockito library to DialerTests's Android.mk

Bug: 24270592

Change-Id: I00d0f75edaecaa85042b136b0d830b5fbb3a0a73