CameraBase: Don't return an sp<> by reference

If the server dies, the binder death callback clears out
the global camera service sp<>, and any current references to it
will become quite unhappy.

Test: Camera CTS passes
Bug: 31992879
AOSP-Change-Id: I2966bed35d0319e3f26e3d4b1b8dc08006a22348

CVE-2017-0544

Change-Id: Ib7ef455366927b0471f8fcabdd5a54e38e375d41
(cherry picked from commit 4b49489c)
(cherry picked from commit c59a656f)