Clean up messy dispatch in bt_hci_bdroid by using new primitives.

Instead of dispatching onto a worker thread by setting bits of a
bitfield and then having the thread pull data from shared data
structures, this code uses the previously defined primitives to
write to a synchronized thread event queue. It's advantageous because
it's more direct dispatch (you specify which function you want to
call on the worker thread instead of mapping bitfields to a function),
it avoids a lot of boilerplate code, function calls have predictable
ordering, and it uses generic infrastructure instead of building a
custom dispatch + synchronized data strucures.

Change-Id: If938e4fd47c8ddf328a29d104151f900ec659588