Merge changes I1bd2a3c5,I3c65d97c,I1fd446a1,I6946791b,Ie1f73bc1, ...

* changes:
  Remove bta_sys_free, it's empty
  No need to bounce BTA_sys_signal_hw_error through main thread
  No need to bounce through BTA_dm_on_hw_error
  Delete bta_sys_sm_execute
  Delete bta_sys state machine, freaking finally
  Inline bta_sys_hw_api_disable
  Replace BTA_SYS_API_DISABLE_EVT with direct call
  Send BTA_SYS_API_DISABLE_EVT from stack manager
  Inline BTA_DisableBluetooth
  Remove now unused bt_startup_thread
  Inline btif_disable_bluetooth
  tBTA_DM_ENABLE status is always successful
  bta_dm_enable is guaranteed mutually exclusive
  Flatten BTA_SYS_EVT_STACK_ENABLED_EVT out of the state matchine
  Pull BTA_SYS_EVT_STACK_ENABLED_EVT out of BTM_reset_complete