Context Hub default HAL: Use std::thread for endpoint callbacks

This will ensure the endpoint callbacks are called after
returning from the original initiating function in another thread.

This CL also updates the VtsAidlHalContextHubTargetTest to handle
the async callbacks.

Bug: 380335353
Change-Id: I29d932f8a4d8989c06cfa6007368a424c963c91f
Flag: TEST_ONLY
Test: atest VtsAidlHalContextHubTargetTest