TrafficMeter: more optimizations
Traffic meter: option to show only while download active (4.3 only)
Will listen for DownloadManager state changes and start/stop traffic
updates depending on whether there are ongoing downloads in progress

Cherry-picked and backported from KitKat branch

Revised by bgcngm:
- updateLocked method (from com.android.providers.downloads.DownloadService class)
has been implemented on Android 4.3, so feature is filtered