Created commands to manage metered networks.

These commands will be used primarily in the CTS tests:

- list metered-networks [BOOLEAN]
- get metered-network ID
- set metered-network ID BOOLEAN

For example, assuming the device has 2 wi-fi networks, and neither is metered:

// list all non-mobile networks
$ adb shell cmd netpolicy list metered-networks
wifi-A;false
wifi-B;false

// set wifi-A metered
$ adb shell cmd netpolicy set metered-network wifi-A true

// get wifi-A status
$ adb shell cmd netpolicy get metered-network wifi-A
true

// list only metered non-mobile networks
$ adb shell cmd netpolicy list metered-networks true
wifi-A;true

BUG: 26685616
Change-Id: I1ec8be4579477448fcb7f0d6cfa6b9cc6887fa97