Implement onNullBinding() in autofill service connection

With this fix, an autofill service which returns null in onBind() will no longer stay bound by system_server and will no longer be able to launch activities from the background.

Flag: EXEMPT security bugfix
Test: sts-tradefed run sts-dynamic-develop -m StsHostTestCases -t android.security.sts.Bug_401545800
Bug:b/401545800
Change-Id: Ieb3e63fe2ac1f440be3d164730ef3110209ec1a6