flimflam: modemmgr: simplify activation

flimflam: modemmgr: simplify activation

Simplify activation by making its implementation be a simple DBUS call
and a handler for the reply.  By simplifying the handling of activate
we can more easily handle errors that are returned by the call to
activate, and we avoid holding a modem object around if cromo dies
during activation (there will be no outstanding task).

BUG=chromium-os:12913
TEST=Run through activation at Verizon twice, use fake-modem to test activation errors

Review URL: http://codereview.chromium.org/6709018

Change-Id: If3fb53c8e5ee8e5082c56f8c5d876713dc07e396
(cherry picked from commit 408078225eeaf501ad69704b4d81963b77fb34b4)

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=da4f16e