fix: Unbind an A11yService from onNullBinding

With this fix, a 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.

Bug: 386950836
Test: Follow steps in the bug to reproduce the issue; observe that BAL
      is no longer allowed.
Test: atest android.security.cts.Bug_386950836
Flag: EXEMPT security bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:26580c52200ad1c6347c8a1eef38db1ac8ce2c68)
Merged-In: Id04111b061881d23346aa90a0b5d08a28bed2c6f
Change-Id: Id04111b061881d23346aa90a0b5d08a28bed2c6f