Tweak default ignore-assets path

Add .DS_Store to the list of ignored files that are silently
ignored (other dot-files are ignored but aapt emits a "(skipping <x>)"
message.)

Also, add a "!" prefix to the *~ pattern for Emacs/Vim/Gedit backup
files.

Finally, move the !*.scc pattern up in front of the .* pattern, such
that it doesn't match the earlier .* pattern (which is verbose, unlike
!*.scc).

Change-Id: Id3e96490f1802486aea8c58366d43e9d413971b8