System bar improvements.

- Updated artwork for buttons and signal/battery meters.
- Layout change: meters on either side of clock
- RSSI for mobile data. The algorithm is more or less the
  same as the one used in the phone status bar. Note that
  mobile data is only shown if Wi-Fi is unavailable.

Bug: 2924643
Change-Id: Idaa0c52422db4a63616475bde96626d1953830b4