Android: Clean up JNI generated code

It's now possible to generate JNI code for constructors and enums
correctly. This CL cleans that up.

Bug: webrtc:8278,webrtc:8551,webrtc:8556
Change-Id: I2284a30139cbb186c80713eb6113eda5659c16ad
Reviewed-on: https://webrtc-review.googlesource.com/25622
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20860}

sdk/android/src/java/org/webrtc/CalledByNative.java

@@ -19,7 +19,7 @@ import java.lang.annotation.Target;
  * @CalledByNative is used by the JNI generator to create the necessary JNI
  * bindings and expose this method to native code.
  */
-@Target(ElementType.METHOD)
+@Target({ElementType.CONSTRUCTOR, ElementType.METHOD})
 @Retention(RetentionPolicy.CLASS)
 @interface CalledByNative {
   /*