Add network usage stats to status bar (1/2) PS2: Get the time delta between updates and use that to show average bytes per second.

Change-Id: I63771220333a32063f675e6fc38c9aadb6de4274

Conflicts:
	core/java/android/provider/Settings.java
	packages/SystemUI/res/layout/status_bar.xml
	packages/SystemUI/res/values/dimens.xml
	packages/SystemUI/res/values/styles.xml

Conflicts:
	core/java/android/provider/Settings.java

Signed-off-by: MNT <m.zilius@gmail.com>
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.

Change-Id: Ia131a0db65c4c762b7f28493e812f9fb21358a27

Signed-off-by: MNT <m.zilius@gmail.com>
Network stats: Hide if there's no traffic (1/2)