Allow AssistDataRequester to fetch AssistContent without AssistStructure

This provides a new request type for AssistDataRequester, ASSIST_CONTEXT_CONTENT. Using this prevents ActivityTaskManagerService from sending the potentially large AssistStructure when it isn't necessary.

Bug: 189164641
Test: Manual (confirmed the new call worked as expected on Pixel 3A)
Change-Id: I5cec15c88c52f8319808aee474cc2351f2fa931a