Improve LegacyTypeTracker.

Keep track of requests as well as of networks that come and go.
This is necessary, for example, to ensure that we pretend that
HIPRI has gone down when the HIPRI request goes away, even
though the underlying cell network is actually completely
unaffected.

Also, ensure that when switching default networks we send
disconnect broadcasts (and do so *before* connect broadcasts, to
maintain the illusion).

Bug: 16610051
Change-Id: Ib3c831387124940156df05b312cc36bc0724373e