Gate hit test behind feature policy

[bialpio]

Hit test should be gated by the same feature policy that the majority of other features already use ("xr-spatial-tracking").

Other changes:

• session needs to reject requests to hit test if it has already ended.

@mounirlamouri, @Manishearth, @thetuvix - please take a look.

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[mounirlamouri]

LGTM