Reapply an upstream OkHttp change to Android for HttpUrl

Manually reapplied change from upstream to address HttpUrl encoding issues.
This change did not apply cleanly. Upstream FormEncodingBuilder has
been refactored. Equivalent changes have been made.

Upstream details:
Comment:
Never throw converting an HttpUrl to a java.net.URI.

Just do arbitrary amounts of transformation. Sigh.

Closes https://github.com/square/okhttp/issues/2116
SHA: d77edcc8905148f18a691be180c4f8f366a5ee1b

Bug: 27590872
Change-Id: Ia12df5dc95c1681c67552b2c7725393508f52c0e