[Autofill Framework] Add a new key for specifying virtual structure type in ViewStructure extra.

Verified the key can be used and assist structure dump contains the
value by testing on a local build, where I added a placeholder value into the ViewStructure extra. Screenshot of assist structure dumped at debug build: https://screenshot.googleplex.com/6CGGfUgu8kVqx6E

Bug: b/331269835
Test: Tested key is dumped locally with a debug build
Change-Id: I22cbf06c2f3ddf7f35b2063f59d72176056c3f27