Don't destroy BTIF Device Management state during shutdown

Don't call btif_dm_cleanup() during stack shutdown stage,
because it will destroy/free internal state that might be used
later in the shutdown process.
Instead, btif_dm_cleanup() should happen during the stack
cleanup stage.

Also, added an unit test to capture the original issue.

Bug: 27856457
Change-Id: I8575537ad744cf4e6770046b779b6a53de93d66e