fiemap_writer: First cut implementation with write() support.

The library is meant to help write to a file using its FIEMAP and the
raw block device.

Bug: 121211685
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21 1073741824
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21 \
      --gtest_filter=FiemapWriter.CheckBlockDevicePath
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21 32768 \
      --gtest_filter=FiemapWriter.CheckWriteError
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21 \
      --gtest_filter=VerifyBlockWritesExt4.CheckWrites
Test: fiemap_writer_test /data/local/tmp/foo.raw /dev/block/sda21 \
      --gtest_filter=VerifyBlockWritesF2fs.CheckWrites

Change-Id: I7b580afddc829c1064ebb52b7a6c7600a2fb2772
Signed-off-by: Sandeep Patil <sspatil@google.com>