Initial Digital Asset Links support (DAL).

This change uses DAL to verify that an app being autofilled is linked to
the web domain set in a node, and fail if the association could not be verified.

It also includes some minor improvements, such as:

- Clears cache on WebView activity.
- Takes an optional url extra in the WebView activity intent.
- Uses FillCallback.onFailure() to report some errors.

Bug: 66414472
Bug: 66417779
Bug: 66900717
Test: manual verification

Change-Id: I2fa96ae61201d7d11aaab168957da2a5685a5764