SurfaceFlinger: Add --static-screen stats

Adds a --static-screen option to dumpsys SurfaceFlinger, which
displays screen-on time broken down by the time between the prior
frame and the current frame. An example dump looks like this:

$ adb shell dumpsys SurfaceFlinger --static-screen
Static screen stats:
  < 1 frames: 12.235 s (3.5%)
  < 2 frames: 29.898 s (8.7%)
  < 3 frames: 15.370 s (4.4%)
  < 4 frames: 13.103 s (3.8%)
  < 5 frames: 15.780 s (4.6%)
  < 6 frames: 2.022 s (0.6%)
  < 7 frames: 0.201 s (0.1%)
  7+ frames: 256.887 s (74.4%)

The buckets are exclusive, so '< 3 frames' covers the interval
[2, 3) frames

Bug: 19543586
Change-Id: I3253a54c23995d25e96016997acedd0775956b60