[Libomptarget][NFCI] Move logic out of PluginAdaptorTy (#86971)

Summary:
This patch removes most of the special handling from the
`PluginAdaptorTy` in preparation for changing this to be the
`GenericPluginTy`. Doing this requires that the OpenMP specific handling
of stuff like device offsets be contained within the OpenMP plugin
manager. Generally this was uninvasive expect for the change to tracking
the offset and size of the used devices. The eaiest way I could think to
do this was to use some maps, which double as indicators for which
plugins have devices active. This should not affect the logic.