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