[Ch20 - chal.2+3 completed] Added delete + context menu

Fixed:
+ Bug: sensor still runs even after the camera activity has completed

Added:
+ Deleted previous file upon taking new picture
+ Context menu to provide deletion of picture upon long click

Note:
+ onLongClickListener must return true so that no further click
processing is to be done ( to avoid conflicts with onClickListner(..))
+ onActionItemClicked ActionMode callback must return true to show
context menu
+ startActionMode(…) must be implemented to establish a context menu
[Ch20 - chal.2+3 fixed] wired correct ID + immediate close

Fixed:
+ Bugs bunny: Wrong item ID specified in onActionItemClick(..) callback.

Added:
+ Immediate close of contextual action bar after deleting