Initialize m_last_obstacle_msg_time with RCL_ROS_TIME (resolves #806)

Description

The line

auto elapsed_time = request_time - m_last_obstacle_msg_time;

in object_collision_estimator_node throws a runtime error when the bounding boxes callback has never been called – in that case, request_time is ROS time and m_last_obstacle_msg_time is system time:

[object_collision_estimator_node_exe-18] terminate called after throwing an instance of 'std::runtime_error'
[object_collision_estimator_node_exe-18]   what():  can't subtract times with different time sources

Background: Before the first callback, it has clock type RCL_SYSTEM_TIME because that's what the default constructor uses. But in that callback, the m_last_obstacle_msg_time is assigned a builtin_interfaces::msg::Time, and that assignment sets the clock type/source to RCL_ROS_TIME. So that's why this error only occurs when no obstacle message has been received.

A fix for that is to change the clock type that this variable has by default.

Notes for reviewer

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