Enable Ethernet tab in data usage

mStateSession is not initialized at the time hasEthernet is called to
set mShowEthernet, so Ethernet tab is not shown at the startup.
In this case, if the device doesn't support mobile network, user cannot
even select it from the context menu.

Bug: 8365419
Change-Id: I441971f18346c34a3adf3f18d9355075d9dce3f0