Hold a wake lock while dispatching network activity events.

Also add new API for determining whether the current data network
is active, and thus better scheduling network operations.  This
API is designed to not be tied to a mobile network -- regardless
of the network, apps can use it to determine whether they should
initiate activity or wait.  On non-mobile networks, it simply always
reports as the network being active.

This changed involved reworking how the idle timers are done so
that we only register an idle timer with the current default
network.  This way, we can know whether we currently expect to
get callbacks about the network being active, or should just always
report that it is active.  (Ultimately we need to be getting this
radio active data from the radio itself.)

Change-Id: Iaf6cc91a960d7542a70b72f87a7db26d12c4ea8e