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.

Change-Id: I13becdd133cccc912b542d48061fcfe9f566099a
diff --git a/Android.mk b/Android.mk
index f0645d8..e7847d8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -383,6 +383,7 @@
     $(ue_libpayload_consumer_exported_static_libraries)
 ue_libupdate_engine_android_exported_shared_libraries := \
     $(ue_libpayload_consumer_exported_shared_libraries) \
+    libandroid \
     libbinder \
     libbinderwrapper \
     libbrillo-binder \
@@ -419,6 +420,7 @@
     daemon.cc \
     daemon_state_android.cc \
     hardware_android.cc \
+    network_selector_android.cc \
     proxy_resolver.cc \
     update_attempter_android.cc \
     update_status_utils.cc \