Call Redirection: unbind service when onBind returns null

The call redirection service does not handle the corner case of
onNullBinding (occurs when onBind returns null). This vulnerability
would give the app that has the call redirection role unintentional
access to launch background activities outside the scope of a call
lifecycle.

Fixes: 273260090
Test: Unit test to ensure we unbind the service on null onBind
Test: CTS for similar assertion
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c3580d96071a7232ce11ad83848d6394b93121d8)
Merged-In: Ib9d44d239833131eb055e83801cb635e8efe0b81
Change-Id: Ib9d44d239833131eb055e83801cb635e8efe0b81