Add AppSearch.java, containing the Document class, base Builder, and Email.

Document is the basic entity to be indexed and queried in AppSearch.
AppSearch.java contains all AppSearch Document types and their Builders.
They are grouped here to avoid potential collisions with other
generically-named classes like Email and Contact).

Bug: 143789408
Test: atest FrameworksCoreTests:android.app.appsearch
Change-Id: I7bb607ad4114451a3db11b9ade9c197da1e6556a