Allow multiple trust prompts to be started per process

[Ken VanDine]

The content-hub-service needs to be able to start prompt sessions for multiple requests. Use cases:

1. Paste Other from the UITK's TextField invokes the clipboard UI as a trust prompt started by content-hub-service
2. Out of process peer picker, we have a new peer picker that doesn't run in the process of the requesting app. It's started in a prompt session by content-hub-service
3. Content picking. The content-hub-service starts apps that provide a content picker in a trust prompt.

All of these use cases require starting the prompt from the content-hub-service process. We would like to do this without stopping other prompts that might already be in use.

Currently we get the following exception:

Invalid prompt session: Error processing request: Cannot start another prompt session
Internal error details: /build/mir-S1BioZ/mir-0.24.1+16.04.20160928/src/server/frontend/session_mediator.cpp(1052): Throw in function virtual void mir::frontend::SessionMediator::start_prompt_session(const mir::protobuf::PromptSessionParameters*, mir::protobuf::Void*, google::protobuf::Closure*)
Dynamic exception type: boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::runtime_error> >
std::exception::what: Cannot start another prompt session

[Ted Gould (ted)]

This also effects URL dispatcher prompts. For instance we have a Bad URL prompt that shows up when someone uses an incorrect URL. It is quite possible for someone to have a bad URL in one app and use another one at the same time.

[Bill Filler (bfiller)]

very important for ubuntu personal use cases

[Daniel van Vugt (vanvugt)]

Duplicate of bug 1494197