Don't leak local reference to BinderProxy.

The local reference created by javaObjectForIBinder() in the death
handling path is never freed, because it's not part of a regular JNI
call. Use a ScopedLocalRef<> to make sure it gets freed when we no
longer need it.

Bug: 148181449
Test: adb shell dumpsys activity binder-proxies shows low dead nodes
Change-Id: I031a46a310a06826bfadd77de7478b4342cf09ab