Update COM Activator class (dotnet/coreclr#22317)

Update COM Activator class (dotnet/coreclr#22317)

* Update COM Activator to use ALCs

* Update tests to now supply an assembly name rather than an binary "path"
  since the COM activator is now using ALCs.

* Add test for non-rooted assembly path.

* Move the hostpolicy mock project to a Common area
Move hostpolicy mock API to CoreCLR test library

* Add test for COM server isolation

* Move ComActivator class and logic to Internal namespace.

Commit migrated from https://github.com/dotnet/coreclr/commit/09963dc2ee182e7e62212efbb205edd9c387961d