cameraservice: Log session index

For tracking camera performance, it is helpful to know the order in
which camera sessions were 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: No functional change.
      `atest CtsCameraTestCases` passes
Change-Id: I5a5accdb2fd3d1de6e7ec31f3e67faaeccb0d3dc