Remove AUTOFILL_TYPE_RICH_CONTENT and related code

This reverts the autofill API changes made in ag/9884022 (only the
autofill-related changes in that CL, not the rest). This is because
non-text content in autofill is now handled via a separate code path
rather than via an autofill type (the new pipe for rich content was
added in ag/12642608).

Bug: 168837034
Test: Manual (image suggestions in augmented autofill)
Test: atest FrameworksCoreTests:AutofillValueTest
Test: atest CtsWidgetTestCases:TextViewOnReceiveContentTest
Change-Id: I261148646d21474ea10662a3e6a5d8b0ba6708f4