Verify apnContext is not null before it gets dereferenced

If this is still null a NPE will be thrown when it is dereferenced later on

Change-Id: I71ec478eb32425ca4242ee1eb366756648935350
DcTracker: verify preferred apn list is not empty before enabling

If a non-null but empty cursor is returned mCanSetPreferApn will be
set to true even though the preferred apn list is empty. Verify the
cursor contains at least one apn before setting mCanSetPreferApn.

Change-Id: I6a66ab334d997d8ced06ea0b596ba3463f4ce85b
Handle DATA_CALL_LIST in DCC and update dataActivity properly

Update mActivity in DCC before notifyDataActivity. Without this change,
the notifyDataActivity was notifying incorrect data activity status
due to race condition between DCT when mActivity was updated and
DCC thread where notifyDataActivity was called.

Change-Id: Iffb7bf369062264dd773338c732bc69318d90b73
Bug: 11596853
(cherry picked from commit f74c31dfe3c9db4cb8e81be014aaac2537c23f51)