Make thread names more meaningful

* Rename threads:
  - module_wrapper -> bt_module_lifecycle_thread
  - btif_a2dp_sink_worker_thread -> bt_a2dp_sink_worker_thread
  - btif_a2dp_source_thread -> bt_a2dp_source_worker_thread
  - bt_jni_workqueue -> bt_jni_thread
  - stack_manager -> bt_stack_manager_thread
  - hci_thread -> bt_hci_thread
  - bt_workqueue -> bt_startup_thread
  - btu_message_loop -> bt_main_thread
* Rename functions:
  - do_in_bta_thread -> do_in_main_thread
  - get_message_loop -> get_main_message_loop
  - post_to_hci_message_loop -> post_to_main_message_loop
* Refactored bta_sys_sendmsg

Bug: 110303473
Test: make, unit test, stream music
Change-Id: I3051f6ea2d33db37af8eebafba7db4d1e113f611