Work on issue #17656716: Unhandled exception in Window Manager

Try to clean up the code paths coming in and out of binder IPCs to
plug any places where we could disrupt the gather flag of a thread,
causing it to keep gathering stack crawls (which is the thing that
is causing our strict mode data to become so large).

We now take care of saving and restoring this state in the core
IPC code path, not at the Java layer.

Change-Id: I73d564778da127bdce00f304225930e7f2318293