Remove dependency on GKI tasks from BTIF media task.

The previous code started a thread and posted a long-running function
to the thread's event loop. In essence, it created a thread and ran
a custom thread main that did its own event handling using GKI_wait.

The new code doesn't use GKI_wait or GKI_send_event at all. Instead,
it uses thread_post and the thread's reactor to enqueue and dispatch
events.