cameraservice: Log session index

For tracking camera performance, it is helpful to know the order in
which camera sessions are created.

This CL adds a sessionIndex field to CameraSessionStats which tracks
the order in which sessions were created. This field is logged by
ICameraServiceProxy.

Bug: 236318755
Test: CtsCameraTestCases pass. No functional change.
Change-Id: I392caf8040a8172d0471ce89860a52348188db6a