Add null check for AclStateChangedHandler

When we receive ACL state change and can't find corresponding
CachedBluetoothDevice, we will trigger onAclConnectStateChanged with
null activeDevice and cause NullPointerException.
Add a null check in AclStateChangedHandler to prevent it.

Bug: 122049350
Test: RunSettingsLibRoboTests
Change-Id: Iaabccc8c4ad60dbae14bbb0f6af655be06bc18ea