Add new StrictMode thread policy - DETECT_UNBUFFERED_IO

This will enable the developers to check if they could replace an unbuffered
I/O operation with a corresponding buffered I/O operation.

The policy can be set as follow:
     StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
        .detectUnbufferedIO().build());

Test: vogar BlockGuardTest
Bug: 3018772
Change-Id: Ib804a8e9980ae40abfd216f8b87c250fd413bd5c
(cherry picked from commit 9b7184a8)w