Set Javadoc encoding (#686)

This avoids issues with javadoc if the platform default happens to be
US-ASCII or some other encoding that does not support i18n characters
in Google Ads API comments.

Author: jradcliff

buildSrc/src/main/groovy/com.google.api-ads.java-conventions.gradle

@@ -63,6 +63,14 @@ jar {
 
 javadoc {
   options.addStringOption('Xdoclint:none', '-quiet')
+  // Sets the input (source) file encoding to UTF-8. The javadoc tool will also
+  // use this for the output (HTML) file encoding. Without this option, the
+  // javadoc tool will use the default encoding for the platform.
+  options.encoding = 'UTF-8'
+}
+
+tasks.withType(JavaCompile) {
+  options.encoding = 'UTF-8'
 }
 
 components.java.withVariantsFromConfiguration(configurations.testFixturesApiElements) { skip() }
@@ -135,10 +143,6 @@ if (project.properties.containsKey("release")) {
   }
 }
 
-tasks.withType(JavaCompile) {
-  options.encoding = 'UTF-8'
-}
-
 dependencies {
   api 'com.google.api:gax'
   api 'com.google.api:gax-grpc'