NetStats: Don't update stats when screen is off

Also moved registering of broadcast receiver and settings
observer to onAttachedToWindow() and unregistering to
onDetachedFromWindow() to avoid these being leaked.

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/NetworkStatsView.java

Change-Id: I077515a0550a8268b2ec4ed09b5bf0f42315733d
Network stats: use number formating based on android code

Uses logic from Formatter.formatFileSize

Change-Id: I9008df0e90fbaf8d733cfd853dc1ef6160cf67f5
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>