update_engine: Break instantiating system policies into their own files

This removes the use of __ANDROID__ for instantiating system
policies (Chrome OS vs. Android) by defining an interface and
implementing the interface in each individual code base.

BUG=none
TEST=FEATURES=test emerge update_engine
Did not build the android side, but hopefully it is correct as the code
change is minimal and problems can get caught in reviews.

Change-Id: I3931a9bed9ee4a9edfba5d712b05d487af1af813
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/1755263
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/update_manager/android_things_policy.cc b/update_manager/android_things_policy.cc
index 26bd0ba..a76ea48 100644
--- a/update_manager/android_things_policy.cc
+++ b/update_manager/android_things_policy.cc
@@ -16,6 +16,7 @@
 
 #include "update_engine/update_manager/android_things_policy.h"
 
+#include <memory>
 #include <string>
 #include <vector>
 
@@ -30,10 +31,15 @@
 using base::Time;
 using chromeos_update_engine::ErrorCode;
 using std::string;
+using std::unique_ptr;
 using std::vector;
 
 namespace chromeos_update_manager {
 
+unique_ptr<Policy> GetSystemPolicy() {
+  return std::make_unique<AndroidThingsPolicy>();
+}
+
 const NextUpdateCheckPolicyConstants
     AndroidThingsPolicy::kNextUpdateCheckPolicyConstants = {
         .timeout_initial_interval = 7 * 60,