Fixed race condition in data connection

Fixed that multiple threads are accessing
APN contexts associated with the data connection.

Test: Telephony sanity tests and unit tests
Bug: 120088861
Merged-In: Iffc840715684f4db78f3635b403e1b2772412099
Change-Id: Iffc840715684f4db78f3635b403e1b2772412099
(cherry picked from commit a7f9dce3)