Set "results source" on RemoteInput intents sent from System UI

If the input comes from RemoteInputView (the user manually typed an
inline reply), the source is set to RemoteInput.SOURCE_FREE_FORM_INPUT.
If the input comes from SmartReplyView (the user tapped on a smart reply
button), the source is set to RemoteInput.SOURCE_CHOICE.

Bug: 67765414
Test: atest RemoteInputViewTest SmartReplyViewTest
Change-Id: Icff46b903a968bd8457624b358d626b457569352