SurfaceFlinger: Add support for continuous dumpsys to file.

- Collect dumpsys to an outfile file when triggered.
- Collect dumpsys before calling Prepare on hwc module in
  each draw cycle. Recollect dumpsys if Commit goes through
  successfully and replace former dumpsys with this.
- Wrap around if file size reaches appx 20 MB.
- Generate output file at /data/misc/display/dumpsys.txt
- Syntax:
      adb shell dumpsys SurfaceFlinger --file [--no-limit]
          --file : Ouput dumpsys to file
          --no-limit : Do not wrap around, keep appending

      Use same command to trigger start and end of dumping.
- Output format:
      | start code | after commit? | time stamp | dump size | dump data |

CRs-Fixed: 947084

Change-Id: Ie520f51c69757aeec88b9400688a7f3271472349