Reduce lock contention in AssetManager.java

Previously AssetManager always acquired a lock to decrement a ref count
but we only really need to do that when the resulting count is zero and
we are going to destroy the native object. This changed the ref count to
an atomic integer and reduces how much we acquire the lock.

Fixes: 375952570
Test: Automatic
Flag: EXEMPT bugfix
Change-Id: Icf5ac14635047758a3cb307168636c7ae0eaa2b9