Network stats: Improved codes

* Format speed text using Formatter.formatFileSize
* Set speed text to empty if speed below 1 KB
* Don't do unnecessary things if network stats is disabled
* Address appropriate task for each intent
* SettingsObserver should only observe settings part

Change-Id: I02f7bbb3f911569d2227e3df21e5c72456a157c4
Dynamic data usage [2/2]

Change-Id: Ibd0067418435442d8cff922faf8ef0f6a5a28761

Conflicts:
	core/java/android/net/NetworkPolicy.java