getsdcardpath, getscreenshotpath, getlastscreenshot

Added a few new functions to envsetup.sh:
- getsdcardpath() returns the sdcard path. This will be more useful
  when multiuser storage complicates the sdcard path.
- getscreenshotpath() returns the path to screenshots
- getlastscreenshot() adb pulls the most recently-taken screenshot

Change-Id: I851145088344dff6f1672f0a423201f3aa3e4390