samples: sensor: qdec: Correct encoder emulation

Use phase shift between the emulated A and B signals equal to the half
of the used period instead of a fixed 1 millisecond, to not enforce
decoders to use relatively frequent sampling just to make use of this
emulation.
Add also a few missing `static` keywords.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>