CameraService: Bump up the maximum number of cameras

 Currently at maximum only two cameras will be
 advertised by CameraService layer. If we need
 to support multiple CameraHal implementations
 (HAL 1.0 vs. HAL 3.0) at the same time, then
 this limit needs to be updated.

Change-Id: Idc9d72da2eecfc2080dcc3f362d3939cebbd17eb