Use non-deprecated method.

getChannel() has been deprecated in favor of getChannelId() and will be
removed soon.

Bug: 135214188
Test: atest com.android.internal.telephony.CarrierServiceStateTrackerTest
Test: atest com.android.internal.telephony.ServiceStateTrackerTest
Test: atest com.android.managedprovisioning.common.NotificationHelperTest
Change-Id: I82d68b8cccdee21329cebe8901b43dde61212588