inputflinger: Initial support for rotary encoders.

This change introduces support for rotary encoder input devices.

We also define a new input source (namely, AINPUT_SOURCE_ROTARY_ENCODER)
and a new axis of input (namely, AXIS_SCROLL), since the rotary encoder
motion doesn't necessarily tie to a horizontal or vertical scroll
motion.

A ROTARY_ENCODER input device class is also introduced, corresponding to
the new input source.

A new input source can be defined as producing rotary encoder motion
events, if its corresponding .idc file contains the following
declaration:

    device.type = rotaryEncoder

Bug: 18707397

Change-Id: I8ccd540908311d1ff44fdfeba81b691895413641
Signed-off-by: Prashant Malani <pmalani@google.com>