[lldb] [Process/FreeBSDRemote] Fix attaching via lldb-server

Fix two bugs that caused attaching to a process in a pre-connected
lldb-server to fail.  These are:

1. Prematurely reporting status in NativeProcessFreeBSD::Attach().
   The SetState() call defaulted to notify the process, and LLGS tried
   to send the stopped packet before the process instance was assigned
   to it.  While at it, add an assert for that in LLGS.

2. Duplicate call to ReinitializeThreads() (via SetupTrace()) that
   overwrote the stopped status in threads.  Now SetupTrace() is called
   directly by NativeProcessFreeBSD::Attach() (not the Factory) in place
   of ReinitializeThreads().

This fixes at least commands/process/attach/TestProcessAttach.py
and python_api/hello_world/TestHelloWorld.py.

Differential Revision: https://reviews.llvm.org/D90525