Initial commit for topshim facade

- Add the entry point for the topshim stack with facade
(bt_topshim_facade).
- So far it works only with Android tree; we want to reduce the
dependency further.  So far it depends on 4 shared_libs defined in
Android.bp.
- Introduce the initial layout of the APIs (facade.proto) and the
corresponding implementation (facade/src/*.rs), and sample client code.
We also have a simple Python unittest to verify the framework.  Note
that we want to run it with mobly next.  The existing code is fully
compatible.
- Introduce the use of Python 3 async to help with event stream.
- Currently we generate grpc python manually through protoc.  We will
migrate to Android Build system (and another script) next.

Tag: #gd-refactor
Bug: 181590011
Test: make and run bt_topshim_facade
Change-Id: Ie0a4e5a7c65dd9812240f24444b787e0d5ba8259