test_vendor_lib: Use base::Thread

This upload starts the replacement of the event loop in HciTransport
with libbase's MessageLoopForIO. A global instance of the new
VendorManager class now manages the trio of previously global objects
(i.e. HciTransport, HciHandler, and BREDRController). Note that the new
libbase event loop structure doesn't actually handle anything right now
and the functionality will come in a future change.

Bug: 21586676
Change-Id: Id74392e2a566e06445eb65269f0142265087d47d