TouchInputMapper: Use a fixed scale factor for external drawing tablets

When using an external drawing tablet with a stylus, we expect there to
be a direct mapping between a subsection of the drawing tablet and the
entire display. In other words, we expect the user to be able to access
all parts of the display from within the drawing tablet surface.

We also expect to use the same scale factor in both directions so that angles
are preserved between the drawing tablet's space and the display space.
Moving a unit north on the drawing tablet should move the pointer by the
same amount on the display as moving a unit east.

External drawing tablets are almost always orientation un-aware. We
ensure that the active area of the drawing tablet is configured so that
the top-left point of that area (0, 0) always corresponds to the
top-left corner of the rotated display space.

Bug: 236798672
Test: atest inputflinger_tests
Test: Wacom Inutos S drawing tablet
Change-Id: I201874d66f276c57a7a7d0e23c14d5d13a2bf08d