Organize imports
First step of atomic deletion of a playlist, makes less calls to AndroidDB

When a deletion of a playlist form GS DB fails, it no more tries to remove it
form Android DB. But when a deletion from Android DB fails, it is not
restored to the GS DB. So the atomicity is not completely fulfilled.

The playlists objects are created only once, at activity creation time. When
changes are made to the playlists' ArrayList, only the adapter is reset.

Other changes:
* Removes deprecated comments
* UnisonDBHelper: Adds is_checked field to playlist table
FIX fetching a playlist from GS database
Atomic insertion/deletion of a playlists in databases

Tracks should be fetched and added to the playlist object (not tested).
UnisonAPI: Updates url for fetching top tags.
Adds API calls to update localId and remove a playlist
Ends up adding/removing playlists from/to android/gs in-app/gs server

SoloPlaylistsActivity: Save/remove menus in onContextSelectedItem. When
removing a playlist from android DB, still show playlist in available ones.
When removed from the available list, playlist is removed from the server.
UnisonDB: Insertion/deletion from databases in atomic way
strings: Adds some error messages
fix null pointer exception introduced while trying to avoid null pointers!
In LibraryService.java
FIX various bugs in playlists management
Merge branch 'master' of https://github.com/lca4/unison-android
FIX onKeyDown() hierarchy

Clicking on physical return button goes back to previous activity, finishing
the leaved activity.
Updates onKeyDown() managment

onKeyDown() is overriden only when needed, else the default OS behaviour is
applied.
merge NFC into master
Merge branch 'NFC'