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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d883efa912c0a39c46437074576cffb7078ac455)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:727898d7b32218f45414d68e84a973734f92a56c)
Merged-In: Ieb3e63fe2ac1f440be3d164730ef3110209ec1a6
Change-Id: Ieb3e63fe2ac1f440be3d164730ef3110209ec1a6