Rework network stats to use proc state for fg/bg.

Switch to using the process state to determine whether a
process should be foreground or background, instead of the
boolean foreground given by the activity manager.

This is for battery save mode, where we can now allow more apps
to havenetwork access: everything whose process state is at
least IMPORTANT_FOREGROUND, which allows music playback
and other use-visible things to continue to have network
access.

Note this also impact the traditional background data disabled
state, where now we allow anything top or better to have
network access.  This automatically includes all persistent
processes, the current top activity, and any other processes
hosting the top activity or being used by the top activity.
So it broadens the set of apps that get network access, but I
think this increases it to a reasonable set of things that may
actually be needed for the foreground app to work correctly.

Change-Id: Icb609a2cea280dc3fa3e83417f478ed77f3685aa
4 files changed