Rebind backup transports only when clearly needed

Significantly narrow the circumstances under which transports
will be re-bound.  In particular, we now do not unbind + rebind
whenever any component in a bound transport's host package changes;
rather, we do so only when the transport component itself has
changed state, or when there is a state change that might cause
a new transport to become available.

Bug 19775237

Change-Id: Ib386875df19ffe9f2d3eb9f9788187338360644a