FROMLIST: binder: use inner lock to protect thread accounting

(from https://patchwork.kernel.org/patch/9817763/

)

Use the inner lock to protect thread accounting fields in
proc structure: max_threads, requested_threads,
requested_threads_started and ready_threads.

Change-Id: I5a17eb68812702f803d4e2806e7887de0b3af18e
Signed-off-by: Todd Kjos <tkjos@google.com>