Swap BTA and HCI queues with message loops

Swapping out the queues with base::MessageLoop allows for an easier refactor
of alarms as we can just replace them with an instance of base::Timer. Also
refactored out the data_dispatcher as most of the code wasn't being used.

Bug: 37245162
Test: Bluetooth sanity test and listening to music
      added net_test_btu_message_loop
Change-Id: I73c359f07a225733dc89f51422a6a24ce807c593