pw_transfer: Set event loop in non-main thread

When the Python pw_transfer.Manager is created, it creates a new asyncio
event loop, which is auto-registered if created in the main thread.
However, if the pw_transfer.Manager is created in a non-main thread,
this event loop is not registered, and the pw_transfer.Manager
constructor will fail with "RuntimeError: There is no current event loop
in thread 'THREAD_NAME'.". Check if in non-main thread to register the
new event loop.

Change-Id: Ia22260a0ecd5d1921957efd6d42f534cfbdd5676
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100415
Pigweed-Auto-Submit: Carlos Chinchilla <[email protected]>
Commit-Queue: Wyatt Hepler <[email protected]>
Reviewed-by: Wyatt Hepler <[email protected]>
Commit-Queue: Carlos Chinchilla <[email protected]>

pw_transfer/py/pw_transfer/client.py

@@ -74,6 +74,9 @@ def __init__(self,
         self._write_stream: Optional[BidirectionalStreamingCall] = None
 
         self._loop = asyncio.new_event_loop()
+        # If constructed from non-main thread, set the event loop.
+        if threading.current_thread() is not threading.main_thread():
+            asyncio.set_event_loop(self._loop)
 
         # Queues are used for communication between the Manager context and the
         # dedicated asyncio transfer thread.