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: I0cb85c6df7f221d841dbd42c9481f8563558e421
Change-Id: I0cb85c6df7f221d841dbd42c9481f8563558e421
(cherry picked from commit 74c62b14)