[Android] Rework multidex and enable multidex for unit_tests_apk.

testing/test.gni

Index: testing/test.gni
diff --git a/testing/test.gni b/testing/test.gni
index 7ce5d85acd27fe28fe7b87bd33283976c7cd6ac6..6739bfebb21ce85f18903e934fe9822658d2f109 100644
--- a/testing/test.gni
+++ b/testing/test.gni
@@ -66,6 +66,7 @@ template("test") {
                              [
                                "android_manifest",
                                "deps",
+                               "enable_multidex",
                                "use_default_launcher",
                                "write_asset_list",
                              ])