fix a potential native crash during bluetooth sutdown

fix a potential native crash during bluetooth sutdown

When bluetooth is initialized which call gki_buffer_init,
the buffer pool is allocated by GKI_os_malloc which is
to call libc malloc function.
When bluetooth is cleanup which call GKI_shutdown,
the buffer pool will be  freed by GKI_os_free
which is to call libc free function.
So if we access the buffer, after the whole buffer pool is freed,
we will get this native crash.
if we call cleanup during bluetooth disabling:
the state is still BTIF_CORE_STATE_DISABLING,
then GKI_shutdown will call before bte_main_disable
( bt_hc_if->cleanup();), which is  before userial thread exit.
Then we may trigger this native crash.

bug:11793775
Change-Id: Ic8ced12848d25f918a28f4d5ba29b803d6ac5bd7