Add unified API for inserting rich content (e.g. pasting an image)

The new callback provides a single API that apps can implement to
support the different ways in which rich content may be inserted.

The API is added to TextView and unifies the following code paths:
* paste from the clipboard (TextView.paste)
* content insertion from the IME (InputConnection.commitContent)
* drag and drop (Editor.onDrop)
* autofill (TextView.autofill)

Corresponding API in support lib: aosp/1200800

Bug: 152068298
Test: Manual and unit tests
  atest FrameworksCoreTests:TextViewRichContentReceiverTest
  atest FrameworksCoreTests:AutofillValueTest
  atest FrameworksCoreTests:TextViewActivityTest
Change-Id: I6e03a398ccb6fa5526d0a282fc114f4e80285099