Android: Update generate_jar_jni rule so it can be used from WebRTC

This CL makes the generate_jar_jni rule behave similarly as
generate_jni; it takes a custom jni_generator_include and
the generated files are placed in the same folder as for
the generate_jni targets.

Bug: webrtc:8278
Change-Id: I5317ff1efead452f275a9a3d415793bde0e344d0
Reviewed-on: https://chromium-review.googlesource.com/757377
Commit-Queue: Magnus Jedvert <magjed@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515843}

build/config/android/rules.gni

@@ -284,11 +284,15 @@ if (enable_java_templates) {
     }
 
     jni_package = invoker.jni_package
-    base_output_dir = "${root_gen_dir}/${target_name}/${jni_package}"
+    base_output_dir = "${target_gen_dir}/${target_name}/${jni_package}"
     jni_output_dir = "${base_output_dir}/jni"
 
+    if (defined(invoker.jni_generator_include)) {
+      jni_generator_include = invoker.jni_generator_include
+    } else {
       jni_generator_include =
           "//base/android/jni_generator/jni_generator_helper.h"
+    }
 
     # TODO(cjhopman): make jni_generator.py support generating jni for multiple
     # .class files from a .jar.