fix: add overlay constructor_overrides and remove stale pdf.yaml

Add ConstructorOverride support to the javagen Overlay struct so
per-class constructor params can be customized without editing
generated spec files.

Use it for android.graphics.Canvas: specgen picks the no-arg
Canvas() constructor, but the useful one is Canvas(Bitmap). The
new graphics_pdf overlay sets the correct constructor_params.

Also remove stale spec/java/pdf.yaml which was a subset of
graphics_pdf.yaml left over from before file-name disambiguation.
Remove manual NewCanvas/NewPaint from graphics_extra.go since
javagen now generates them from the spec.

Author: xaionaro@dx.center