Mobile Network Battery Saver Mode

When idle (screen off) switching to 2G can help extend
battery life as 2G generally uses less power than 3G or LTE.

    + 30 seconds after screen turns off, set network type to 2G
    + If a music player is active (playing), do not set network to 2G
    + When device is unlocked, reset to orignial network type

Settings -> More -> Mobile Networks -> Battery saving mode

PS2: Patchset by Alexander Pohl (@OldDroid):
     * If wifi is enabled do not change networkmode.
     * If heavy traffic do not set networkmode to 2G.
PS3: Restore correct indenting.
PS4: Rebase with latest source.

Change-Id: Id3eb75dd0e15598b79eb53d5780782546474b2ae