First steps to save playlist to android database
Partial implementation of playlist storage in android database
Adds playlist to android sqlite DB, updates tag management

* Playlist not stored in GS in-app database for now
* AbstractFragmentActivity subclasses no more need to explicitly specify a
  tag, it is automatically assigned on creation
* Creates Const.Integers class
Creates schema for playlist table in gs in-app sqlite database

Replaces table specific [TABLE]_C_ID field by generic C_ID field, since it is
common to every table.
Completes storing playlist to android and gs in-app db (not tested)

Other various minor changes
Updates field names of playlist JSONStruct and Playlist class

such that they fit the ones of the server responses.
Adds and removes playlists to/from android/GS databases

Updates fields of p|Playlist(s) database and object, creates indexes on
playlists
Adding/removing playlists with its tracks through the app works

* MusicItem: new constructor with playOrder parameter
* ConstDB: adds local_update_time field, renames "playlists" table into
  "playlist"
* UnisonDBHelper: updates playlist schema
* Playlist: adds fields and updates builder
* AndroidDB: FIX one-track only insertion