extract anchor href and image src separately

Long pressing a link may return an anchor, an image, or an
image in an anchor. Sometimes we want one, sometimes the other.

Make both available so the context menu can choose.

Update the documentation in WebView.java

Companion changes are in external/webkit, packages/apps/Browser

bug:3282745
bug:3263340

Change-Id: I9917a2c69d26601a230051f2e95c9464ba6155e9