PointerIcon: Treat 0 as a valid type index for system icons

PointerIcon.TYPE_ALIAS maps to the style type index of 0, which we
incorrectly treat as "invalid" in the code. This means we can never load
the TYPE_ALIAS system icon correctly.

Fix this by using an index of -1 to represent an invalid index.

Fixes: 339233006
Test: presubmit
Change-Id: I28cb4ce53dafa9de88959ba425889fec42d1e722