Traffic meter: received bytes by time elapsed rather than fixed value

Also:

- avoid unnecessary object initialization
- improved display logic for B/KB/MB

I'll probably further improve by disabling everything if traffic meter is disabled. As it is, the traffic meter runs (but does not display) even if it's disabled. Also, the update logic is unnecessarily circuitous…
Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/Traffic.java

Change-Id: I663e3fbcf53c43277649a2bd7acad53e83fbb91c