Disable class unloading when tracing is active

Tracing keeps a lot of pointers to ArtMethods that could belong to
classes that we are going to unload. A quick fix for this is to
disable class unloading when tracing is active.

Bug: 24414774
Bug: 22720414
Change-Id: Ia5619cbd7c9fd558eaa2a5000871d287213c4a76
4 files changed