Copter: move RC_Channels_Copter::save_trim to Copter class

[rmackay9]

Feature Request

As part of reviewing PR #32233 @peterbarker and @rmackay9 both thought that it would be best if this was moved to the Copter class. The means the code itself should be moved from RC_Channel_Copter to Copter.cpp (I think)

This shouldn't be any change in behaviour but it should be at least lightly tested in SITL.