speech: Re-add on-device speech recognition settings entry

After `Settings > System > Languages & input` was separated into
`Settings > System > Languages` and `Settings > System > Keyboard`,
`On-device speech recognition` was lost. This CL adds it in the same
`Speech` preference category where it was before
and moves the category to `Settings > System > Languages`.

Bug: 278279110
Test: Manual, already existing robotests
Change-Id: I040c284dd3596f35ceacb6980ff89e611e29971c