docs: Updated #ACTION_REQUEST_SET_AUTOFILL_SERVICE description

Making clear that `ACTION_REQUEST_SET_AUTOFILL_SERVICE` is an activity
action, not a broadcast action.
Test: Ran `make ds-docs -j16 and staged the change at
    http://go/dac-stage/reference/android/provider/Settings.html#ACTION_REQUEST_SET_AUTOFILL_SERVICE

BUG: 62470123