Ensure that binding state is false when handling dead binder

When a binding is disconnected, it is expected that binderDied() is
called after onBindingDied(). But, there exists a corner case where
the order is reversed. In this case, binderDied() cancels the
scheduling of MSG_UNBIND message, which blocks the chance of changing
mBinding value as false. As a results, mBinding value remains as true
even though the binding is disconnected. This would make the service
as unresponsive when a client tries reconnecting to the service.

This change ensures that mBinding becomes false even though binderDied()
is called before onBindingDied().

Bug: b/162753393
Test: compile & verify basic functions working
Change-Id: I66c5b130674a4355123fbac27f574d0d081e7b63