Add meteredOverride field to WifiConfiguration.

This stores a user's metered selection. This will be set by
Settings -> Data Usage -> Network restrictions and used in conjunction
with WifiConfiguration.meteredHint to properly track a Wifi network's
metered state.

Bug: 33106464
Bug: 26545374
Test: runtest --path
frameworks/opt/net/wifi/tests/wifitests/src/com/android/server/wifi/util/XmlUtilTest.java
runtest --path
frameworks/opt/net/wifi/tests/wifitests/src/com/android/server/wifi/WifiStateMachineTest.java

Change-Id: I1a378ef2946c1b5b6091bba379f2ab48a15d29d8