MobileData slice should be null if not-actionable

In cases where MobileData should not be changed:
- Airplane mode
- No data subscriptions

we will return a null slice, rather than a slice with a no-op intent
attached as a primary action. The problem with the no-op intent is that
Slices assumes all actions are by definition, actionable, and the the
TalkBack description announces that the item is clickable, which it
really isn't.

We will in the future investigate disabled actions on Slices, but this
is the short-term fix.

Fixes: 132924748
Test: Robolectric
Test: Panel tester app
Change-Id: I1d62af32fe2dd985f0b52ea4188651e76f9c90ec