Mark addWindowTargetLocked as const

While figuring out how to refactor 'findTouchedWindowTargetsLocked',
it's helpful to understand where the state is being modified in that
function.
Let's mark addWindowTargetLocked as const to help with the analysis.

Bug: 211379801
Test: build
Change-Id: Ifd0a67bc915996e5401f3d624958660a8680ad19