DeskClock: Refactor to ask user for permissions

* In preparation for T's POST_NOTIFICATION permission, we need
  to ask the user to grant it (if he revoked it, since we pregrant it)
  or to finish the app since it will not work anyway
* Use the chance to show the rationale for the power-off permission
  already

Change-Id: I117586781e03320a3662d26fbe1ee85950f1fd50