Refactored whitelist restrict background uids.

On Android N, the list of UIDs whitelisted was kept in a separate array
and XML tag, while in reality it should be a new UID
policy (POLICY_ALLOW_METERED_BACKGROUND).

This change refactors NetworkPolicyManagerService to use the new UID
policy, although without removing any of its existing methods (but
marking them as deprecated).

Test: m -j32 FrameworksServicesTests && adb install -r -g ${ANDROID_PRODUCT_OUT}/data/app/FrameworksServicesTests/FrameworksServicesTests.apk && adb shell am instrument -e class "com.android.server.NetworkPolicyManagerServiceTest" -w "com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner"

Test: cts-tradefed run commandAndExit cts --skip-device-info --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker --skip-preconditions -m CtsHostsideNetworkTests --abi armeabi-v7a -t com.android.cts.net.HostsideRestrictBackgroundNetworkTests

BUG: 28791717

Change-Id: I39869efda554ca0b736dd2380e439474f91dfbe6