WebView: deprecate accidentally-exposed constructors

This deprecates public-exposed constructors. These constructors were
exposed by accident. These classes shouldn't be instantiated by
applications, but should only be instantiated by WebView.

In some cases, the app should get a singleton instance using
a #getInstance method. In these cases, we document this explicitly in
the deprecation note.

Bug: 110807530
Test: make docs, manually verify docs look good.
Change-Id: Ibe73b3399c9ced0cf4fbb01e1df13564476df252