Mark AndroidNetworkLibrary as @MainDex

When network service runs out of process, it uses a privileged
process similar to the GPU process. The network process needs
access to AndroidNetworkLibrary, which was not in the main dex.
This was causing many failures when running tests with network
service turned on on KitKat devices, see example failures:
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-kitkat-arm-rel/113359

The failures are throwing a ClassNotFoundException for
AndroidNetworkLibrary.

Bug: 827531
Change-Id: I9003a8469719e06cb5a05482d5b7cf24980a32b0
Reviewed-on: https://chromium-review.googlesource.com/c/1306344Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603967}

net/android/java/src/org/chromium/net/AndroidNetworkLibrary.java

@@ -27,6 +27,7 @@ import org.chromium.base.ContextUtils;
 import org.chromium.base.VisibleForTesting;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.annotations.CalledByNativeUnchecked;
+import org.chromium.base.annotations.MainDex;
 import org.chromium.base.compat.ApiHelperForM;
 
 import java.io.FileDescriptor;
@@ -51,8 +52,8 @@ import java.util.List;
 /**
  * This class implements net utilities required by the net component.
  */
+@MainDex
 class AndroidNetworkLibrary {
-
     private static final String TAG = "AndroidNetworkLibrary";
 
     // Cached Method for LinkProperties.isPrivateDnsActive().