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: I8cf519f40ddfe4fd00d99b82fdb88dc069611787
Signed-off-by: Todd Kjos <tkjos@google.com>