Bluetooth: Add a second thread for the scheduler

Test: service call bluetooth_manager 6 # (Stop Bluetooth)
      /data/nativetest64/VtsHalBluetoothV1_0TargetTest/\
          VtsHalBluetoothV1_0TargetTest
Bug: 38178265, 34461621
Change-Id: Ia8f6683ca47c9deac43d1047846caee5688f1eca
(cherry picked from commit 74c60136)