Prevent deadlock in binderDied in validation layer

In binderDied, we hold the validation layer lock as we call unloadModel,
which blocks on callbacks, violating our lock ordering.

As a temporary fix, cache the currently loaded models, and unload them.

Bug: 237602275
Test: Compiles
Change-Id: I5091cb152765ea1f78958108a331d75951b8b717