[android] Keep Services in the main dex.

TBR=wychen@chromium.org,yusufo@chromium.org,mlamouri@chromium.org

Bug: 725534
Change-Id: Ifc0696d08d82d0ea41fc96c7ef09e54cf161b3f2
Reviewed-on: https://chromium-review.googlesource.com/512365Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#473946}

build/android/main_dex_classes.flags

@@ -18,6 +18,10 @@
   *;
 }
 
+-keep public class * extends android.app.Service {
+  <init>();
+}
+
 # Used by tests for secondary dex extraction.
 -keep class android.support.v4.content.ContextCompat {
   *;