DC: add a real tasker

for now we only offered automatic fstrimming at specified intervals.
this time, we add a more "serious" tasker.

it lets you add, delete, enable and disable tasks.
for now we add a screen off and on category as event, when the actions
should get executed.
as actions we add changing cpu min and max frequency, cpu governor and
i/o scheduler.

more to come!

Change-Id: I18d0133017dfd894306f722dc390ea329bf25a56