cm: Do not build CMFileManager

Until CMFileManager is revamped or a suitable replacement is written, do
not include it in general builds. Basic storage activities can still be
completed with Android's built-in storage management:

1) Use the Downloads launcher
2) Settings > Storage (select volume) > Explore

Change-Id: Ic9c4eb2d0347332c4631d55c480b87cbc131116c