Set metadata retriever thread group to the caller's group.

This patch modifies the native binder interface to the metadata
retriever to pass the caller's thread group across the binder
interface. On the server side, the thread scheduler group is
set to the caller's scheduler group temporarily and restored
after the request has completed. This patch also reverts a
previous patch where the priority of the thread was forced to
a low priority foreground thread.

This should give apps more control over the priority of their
metadata retrieval, particularly allow background process to
run without hogging the CPU.