extmod/nimble: Clarify active state and check for active in all methods.

This commit ensures that the BLE stack is active before allowing operations
that may otherwise crash if it's not active.  It also clarifies the state
better (adding the "stopping" state) and renames mp_bluetooth_is_enabled to
the more self-explanatory mp_bluetooth_is_active.

extmod/btstack/modbluetooth_btstack.c

@@ -271,7 +271,7 @@ void mp_bluetooth_deinit(void) {
     MP_STATE_PORT(bluetooth_btstack_root_pointers) = NULL;
 }
 
-bool mp_bluetooth_is_enabled(void) {
+bool mp_bluetooth_is_active(void) {
     return mp_bluetooth_btstack_state == MP_BLUETOOTH_BTSTACK_STATE_ACTIVE;
 }