[Android] Rework multidex and enable multidex for unit_tests_apk.

build/config/android/rules.gni

Index: build/config/android/rules.gni
diff --git a/build/config/android/rules.gni b/build/config/android/rules.gni
index da9a8536f3ed4f1b791461e5319d06169b2ddb2e..fb06814d41e32c41a82f23269fe4fe5361053d58 100644
--- a/build/config/android/rules.gni
+++ b/build/config/android/rules.gni
@@ -1484,6 +1484,21 @@ template("android_apk") {
     _srcjar_deps += [ ":${_template_name}__native_libraries_java" ]
   }
 
+  if (!defined(invoker.apk_under_test)) {
+    java_cpp_template("${_template_name}__multidex_config_java") {
+      package_name = "org/chromium/base/multidex"
+      sources = [
+        "//base/android/java/templates/ChromiumMultiDex.template",
+      ]
+
+      defines = []
+      if (enable_multidex) {
+        defines += [ "ENABLE_MULTIDEX" ]
+      }
+    }
+    _srcjar_deps += [ ":${_template_name}__multidex_config_java" ]
+  }
+
   java_target = "${_template_name}__java"
   java_library_impl(java_target) {
     forward_variables_from(invoker, [ "run_findbugs" ])