Register for kernel global data usage alerts.

Instead of polling every 15 minutes, register for alerts that trigger
when system-wide traffic passes a threshold.  Still mixed with polling
to persist UID stats, but relaxed to 30 minutes.  Currently watches
for every 512kB.

Make persistence decision separately for network versus UID, and use
total delta bytes when making decision.  Use light bootstrap during
systemReady() instead of heavy poll, which had been force-loading all
UID data unnecessarily.

Bug: 5023631
Change-Id: I04b723d6c4bf872fb1028071122dba66a8e1b576