android: Protect against null callbacks

According to the Android camera HAL C interface documentation, the
camera service is supposed to set callbacks after initializing the HAL
and calling get_number_of_cameras(), before any other calls to the
module. We rely on this behaviour and use callbacks unconditionally,
which would lead to a crash if the camera service behaved incorrectly.

While the camera service isn't supposed to behave incorrectly,
gracefully handling the error when opening cameras isn't costly, and
provides better diagnostic than a crash.

While at it, removed an unneeded [[maybe_unused]] attribute.

Reported-by: Coverity CID=298638
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Umang Jain <email@uajain.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/android/camera3_hal.cpp

@@ -32,7 +32,7 @@ static int hal_get_camera_info(int id, struct camera_info *info)
 	return cameraManager.getCameraInfo(id, info);
 }
 
-static int hal_set_callbacks([[maybe_unused]] const camera_module_callbacks_t *callbacks)
+static int hal_set_callbacks(const camera_module_callbacks_t *callbacks)
 {
 	cameraManager.setCallbacks(callbacks);