Avoid a non-root monitor vector in Dbg::GetOwnedMonitors().

Following up on CL 96571, remove the need to use stack-scoped vectors,
one of which is a non-root vector that holds Object*, which could
cause a GC bug.

Bug: 12687968
Change-Id: I8582fe6bfe71bae7018dbf844fe75ebe989b6535
1 file changed