NATT Keepalive Unit Test

-Fix a bug that DC was registering itself
 twice for keepalive status notifications.

-Verify that callbacks register/unregister
 when entering/exiting DC_ACTIVE state
-Verify that KA sessions are cached by a map
 of slotId and sessionHandle.
-Verify that start/stop requests are sent to the
 RIL as needed.
-Verify that KA sessions are cleaned up after a
 call to stop()
-Verify that a failed keepalive is cleaned up and
 the records are either not created or are created
 and then deleted depending on the error scenario.

Bug: 72417084
Test: runtest -x DataConnectionTest.java
Change-Id: I113e170fe20b9bdd74c2670109b8271f2114dab1