Add strings for TV standby function

After a long time of no user activity the device should go to standby
mode, even if eg. video is playing (wakelocks are held).
In this CL, only required strings for this feature are added.
Functionality follows in another CL.

Bug: 137633812
Test: build
Change-Id: Ieb36c940238bea611723763148cc56daab00500c