Adding idle maintenance service.

It is beneficial that there is a mechanism on the platform
to notify applications whether it is safe to perform somehow
expensive operations while the user is not using the device.
Thus, user experience will not be degraded. An example is
discarding of unused blocks on a mounted file system instead
of doing this on every write operation.

bug:8056794

Change-Id: I708bad9d3ce6c8f1d5a1c05c0abf46f81a3d464b