Fix for NPE in DatagramReceiver

Messaging app crash and send multiple ACK for the same received
datagram. When the second ACK come, the datagram does not
exist in DatagramReceiver's cache. DatagramReceiver try to retrieve that
datagram and leads to NPE.

Bug: 311061018
Test: SMS, MMS, call with live network
atest DatagramReceiverTest
atest SatelliteManagerTestOnMockService

Change-Id: I7c7ae3e9f23f458340e28c7579148c91b6b2ac20