[feedback] define ToString for GetScreenshotResponse for better debugging

when any of FeedbackAgentTest.GetScreenshot_* fails, it is useful to see a string representation rather than the byte string
before:
Expected: has 4 elements and there exists some permutation of elements such that:
 - element #0 matches get screenshot response @0x50cb257a5d78 8-byte object <90-42 7F-DE D5-0D 00-00>, and
 - element #1 matches get screenshot response @0x50cb257a5d70 8-byte object <90-49 7F-DE D5-0D 00-00>, and
 - element #2 matches get screenshot response @0x50cb257a5d68 8-byte object <00-00 00-00 00-00 00-00>, and
 - element #3 matches get screenshot response @0x50cb257a5d68 8-byte object <00-00 00-00 00-00 00-00>
  Actual: { 8-byte object <50-46 7F-DE D5-0D 00-00>, 8-byte object
  <90-46 7F-DE D5-0D 00-00>, 8-byte object <00-00 00-00 00-00 00-00> },
  which has 3 elements

after:
Expected: has 4 elements and there exists some permutation of elements such that:
 - element #0 matches a 10 x 10 screenshot, and
 - element #1 matches a 20 x 20 screenshot, and
 - element #2 matches no screenshot, and
 - element #3 matches no screenshot
  Actual: { a 10 x 10 screenshot, a 20 x 20 screenshot, no screenshot },
  which has 3 elements

FLK-177 #comment

Change-Id: I55cfb734fd5ae3f68c7973aa94d037df4b0254d8