Removes unused methods, avoids usage of mMAinActivity from AbstractPlayer

* AbstractMenu
Adds FLAG_ACTIVITY_REORDER_TO_FRONT for solo mode

* *PlayerFragments
Each specialization uses his own HostActivity instead of Abstract.MainActivity
(avoids explicit castings)
Moves playlist mangement from Abstract to Groups player

* Playlist management
The whole logic about handling a playlist (pick next/prev track, etc.) is
moved to the specialized implementation. Required method (next/prev, etc.)
from Abstract are thus set as abstract.

* Getters/setters
Comments unnessary getters and setters, replaced by direct access to instance
variables.
Oragnizes imports and formats project
Displays only playlists created with GS

* SoloPlaylistsLocalFragment
Displays only the playlists created with GS.