Fix JdkObsolete Error Prone errors by using Charset overloads

The newer Error Prone pulled in with the Gerrit rules_java 9.8.0 bump
reports JdkObsolete for URLEncoder.encode(String, String). Switch to
the Charset overload and drop the now-dead UnsupportedEncodingException
try/catch blocks and imports.

Change-Id: Ib32a58f36e85b1692bd555653f5a9cf991a5d367