surface: Make HellfireLauncher reflect remaining missiles in graphics.

Also moved definition of Hellfire ammunition count from Apache's member
to HellfireLauncher's member.
surface: Make HydraRocketLauncher have ammunition count on its own.

Just like HellfireLauncher.
In order to achieve similar result to HellfireLauncher, we made
HydraRocketLauncher derive a common base class with  HellfireLauncher.