Fix toasts message is not displayed when cancel BT pairing request

Before this CL, Bluetooth EventManager did not handle the error
reason that was called UNBOND_REASON_REMOVED. If UNBOND_REASON_REMOVED
is sended when cancel BT pairing, the error toast will not display.

This CL will show error toast when received UNBOND_REASON_REMOVED.

Bug: 173165769
Test: make -j42 RunSettingsLibRoboTests
Change-Id: I5c75c17ebe204c8b9e5f1ff18c39ef8142e009eb