LibreOffice4Android: Disable thumbnails so the application can run

Commented out the piece of code that invokes the creation of
thumbnails, because it crashes after the call to native code.

Once LibreOffice4Android is merged with the LOKit-based viewer, it
will be the moment to fix the thumbnails properly.

Change-Id: I6dd38c6f91008d41905d3c4a856899e9ec81bd6b
LibreOffice4Android: Remove dependency from actionbarsherlock

Replace with the native Android action bar since API 11.

Change-Id: Ia332b51cfea084072e8d0362702a359b8f09adea
LibreOffice4Android: Fix NullPointerException

The member fileNames was being correctly set in createUI() but it was
being reset at openDirectory(). The only purpose of this member was
sending the file name to open() method, but we can rewrite open() to
accept File objects so fileNames became useless and was removed.

Change-Id: I944be8ff13d221573a5a7accf0081c30872c62f0