[Android] Rework multidex and enable multidex for unit_tests_apk. (RELAND 2)

this is a reland of https://codereview.chromium.org/1590243003/

BUG=272790
[email protected],[email protected],[email protected]

Review URL: https://codereview.chromium.org/1594653002

Cr-Commit-Position: refs/heads/master@{#369906}
diff --git a/testing/test.gni b/testing/test.gni
index 7ce5d85a..6739bfe 100644
--- a/testing/test.gni
+++ b/testing/test.gni
@@ -66,6 +66,7 @@
                              [
                                "android_manifest",
                                "deps",
+                               "enable_multidex",
                                "use_default_launcher",
                                "write_asset_list",
                              ])