android: don't let proguard mark ChromePublicForTest classes as final.

Change-Id: I6bfcf3efbe4dbcfa769afb77e732050f6527703c
Reviewed-on: https://chromium-review.googlesource.com/1199766Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588132}

chrome/android/java/apk_for_test.flags

@@ -8,3 +8,6 @@
 # Doing so triggers an obfuscation error in chrome_public_test_apk, which has a
 # -keep for **.test.**.
 -keepnames class **.test.** { *; }
+
+# Do not mark classes as final, as doing so may interfere with mocking.
+-optimizations !class/marking/final