Some StrictMode API changes.

* rename setThreadBlockingPolicy to setThreadPolicy (opens the way to
  using StrictMode for non-blocking-related things in the future?)

* add allowThreadDiskWrites() and allowThreadDiskReads() to modify the
  current policy mask and return the old one.  this will allow turning
  off part of StrictMode during certain regions of code.  (for
  instance, writing to disk in Activity onPause...)

Change-Id: Ia1878153713f79299971fdab567fa15b3cb9d56c