Dialer: Move assertion to top of method

* Instead of using the number unchecked in various places and fail
  later when the value is null, move the assertion to the top of the
  method so we don't have to do all the work in between when we fail
  it anyway
* Also change the occurances of getPhoneNumber() so we don't have to
  call into that various times instead of once

Change-Id: Ibf7ab971dc1ee65e3d8bcc7668e9f9c8c3a30651