Allow to set the network used for an update attempt.

When triggering an update attempt in Android, we can force all the
traffic to go through a given network by passing a NETWORK_ID=<id>
in the extra key-value pair field.

Bug: 29921093
TEST=Called update_engine_client passing NETWORK_ID.

(cherry picked from commit 87792ea818a63f2d246bbba2b7429b85ee8d669f)

Change-Id: I0e8a245961788c75106832e9fea8b07cd894f22d
diff --git a/Android.mk b/Android.mk
index e7cd431..3bca905 100644
--- a/Android.mk
+++ b/Android.mk
@@ -374,6 +374,7 @@
     $(ue_libpayload_consumer_exported_static_libraries)
 ue_libupdate_engine_android_exported_shared_libraries := \
     $(ue_libpayload_consumer_exported_shared_libraries) \
+    libandroid \
     libbinder \
     libbinderwrapper \
     libbrillo-binder \
@@ -410,6 +411,7 @@
     daemon.cc \
     daemon_state_android.cc \
     hardware_android.cc \
+    network_selector_android.cc \
     proxy_resolver.cc \
     update_attempter_android.cc \
     update_status_utils.cc \