Default to keyboard input modality.

Was previously defaulted to voice input modality.
Later there will be a user preference that we defer to.

Bug: b:110174827
Change-Id: I1a0eac49f76a53145589c0095df82b4a2d673d03
Reviewed-on: https://chromium-review.googlesource.com/1100300Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#567463}

ash/assistant/assistant_controller.cc

@@ -195,7 +195,7 @@ void AssistantController::OnInteractionStateChanged(
   assistant_->StopActiveInteraction();
 
   assistant_interaction_model_.ClearInteraction();
-  assistant_interaction_model_.SetInputModality(InputModality::kVoice);
+  assistant_interaction_model_.SetInputModality(InputModality::kKeyboard);
 }
 
 void AssistantController::OnHighlighterEnabledChanged(

ash/assistant/model/assistant_interaction_model.h

@@ -121,7 +121,7 @@ class AssistantInteractionModel {
   void NotifySpeechLevelChanged(float speech_level_db);
 
   InteractionState interaction_state_ = InteractionState::kInactive;
-  InputModality input_modality_ = InputModality::kVoice;
+  InputModality input_modality_ = InputModality::kKeyboard;
   MicState mic_state_ = MicState::kClosed;
   std::unique_ptr<AssistantQuery> query_;
   std::vector<AssistantSuggestionPtr> suggestions_;