Fix BluetoothManager and add integration test

Fix some deadlock issues.
 - Fix some timer deadlock issue in aosp/1727697.
 - Fix a deadlock where we send to tx from rx context, by increasing
 mpsc channel size to at least 2, and use send_timeout() instead of
 send().

Use async for some state machine fn to allow waiting for mutex lock.

Add a python script to do some basic testing.

In next iteration, we will test DBUS callbacks, and handle multi-hci
case better.

Tag: #floss
Test: btmanagerd_test.py with btmanagerd running
Bug: 190012236
Change-Id: Ia8fe6c0f36105657577a5e458787f39dc3bd69b7