Listen for retry intents.

Re-adding code stripped out during the NetworkAgent changes.  Going back
to having ApnContexts pre-initialized at startup since they are no longer
NetworkAgents.  This also lets us preregister for each of their retry alarms.

Undo of damage done by: https://googleplex-android-review.googlesource.com/#/c/476836/

bug:15563289
Change-Id: I6f8855eb24d913c73fef647d4219ae7041216e2b