Spoof touch capabilities + properties: support touch with maximum number of touch points or none of it [per OS]

Original:

We disabled dom.w3c_touch_events.enabled in legacy/trac#10286 (moved) as a defense-in-depth against fingerprinting while providing a patch to neuter the risk here as well. The patch got upstreamed in https://bugzilla.mozilla.org/show_bug.cgi?id=1382499 and made it into Firefox 60 ESR.

We should think about whether we should enable the preference again for desktop platforms. We start doing that for Android with legacy/trac#27256 (moved).

Edit: We should go with 'Option B' as outlined in @thorin's post here: #28535 (comment 2848069)