Rename Enterprise Network Preference to Preferential network service

Rename the logging constant for the "Enterprise Network Preference" to
"Preferential Network Service".

Bug: 183663019
Test: atest FrameworksServicesTests:DevicePolicyManagerTest android.admin.cts.DevicePolicyManagerTest#testSetGetPreferentialNetworkServiceEnabled  com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testSetGetPreferentialNetworkServiceStatus com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testSetPreferentialNetworkServiceStatusLogged
Change-Id: I24508b42b3820091760c372d18a9f6157cd53d50