Run background threads associated with filtering at background priority.
Run background threads associated with filtering at background priority.

This improves the typing responsiveness in the browser a lot, and should
help out in all the other cases in the UI where we use a filter thread.

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base