Add the `btstack` crate

The `btstack` crate contains the floss API implementations independent
of any RPC projection. This patch adds the basic skeleton that
implements the first simplest methods.

Bug: 186492781
Tag: #floss
Test: manual - tested with D-Bus integration in the next patch
Change-Id: I10ff16d7e6cfcb599a3b356c58d7af15bcaa61d3