Add unit tests for HintSessionWrapper

This patch adds several unit tests for HintSessionWrapper, including
one specifically for a recent race condition memory leak.

It also refactors HintSessionWrapper to better support testing, and
creates a macro to shorten the dlsym bindings.

Bug: 299541959
Test: hwuiunit

Change-Id: I05ed15cdbd157b109667563d30627cfc4ae83777