Updater: Check if network is really metered

Instead of assuming so, based on whether user is connected over
a WiFi or Ethernet connection, simply use the proper API.

Use default value of PREF_MOBILE_DATA_WARNING, if it had been
already set.

Change-Id: Ie4fa0301df7f2cebf94c967fc188acc54b2ce71e
Signed-off-by: althafvly <althafvly@gmail.com>