decoder/encoder: simplification of code.

- Useless tests removed
- Bit operation changed to more readable ones
- Handle correctly mask to decode signal on 1 byte or in-between bytes more clearly.
- using static_cast now instead of C casting method.
- Avoid manual vector initialization and using default constructors instead.
- Avoid using intermediate variables when this isn't necessary.

Bug-AGL: SPEC-2988

Change-Id: I049d65f460109772b57df7572bdac8e6500242e0
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>