Remove duplicate code from ActionBarHelper. Add export in multiselect.

---------------

In relation to #390
I know the edit-key branch has variated but it should be fairly easy to change it and move the buttons to the OptionsMenu like in the edit-key branch if you later decide to proceed that way. I can do it if you want. :) 

---------------

In relation to #330 
This is now working. I also added an export button in the multiselect menu both in KeyListSecretFragment and KeyListPublicFragment.  I left the old Export All in the KeyListPublicActivity and KeyListSecretActivity because in android < 3 the multiselect list still doesn't work. 
I tried to implement the export via rowIds as simply as I could as to avoid huge refactoring of the code.

I tested the export function by exporting and importing back again keys in the following situations:
- When viewing a public key in ViewKeyActivity (export of a single key works like before)
- When viewing a secret key in EditKeyActivity (export of a single secret key works like before)
- When exporting all the keys both in KeyListPublicActivity and KeyListSecretActivity (works like before)
- When exporting any number of keys in KeyListSecretFragment and KeyListPublicFragment (works as expected). 

---------------

Hope it's ok, let me know if I forgot something. :)