saveFrame - save file in gallery(sdcard) instead of /data

Prior to this commit, saveFile() was saving data in /data partition. Saving something to this location is trivial to the end user. 
Instead, we should save the image in sdcard and update the image in Gallery.
Fixes : https://github.com/processing/processing-android/issues/56

Signed-off-by: Umair Khan <omerjerk@gmail.com>