Misc fix for bluetooth host build

- Use hci_layer_linux for host
- No-op for some functions for host
- Don't crash if we cannot enable real time thread.  Note that we may
need sudo to do this; or this might be unsupported by kernel.  Make the
stack more generic in this case.

Tag: #gd-refactor
Bug: 181590011
Test: make and run bt_topshim_facade
Change-Id: I05bb8245f8f688ea1ac73608707de962b0ee7a4b