Update the payload public key location in Android.

In Android builds, the payload public key is installed in a different
location.

Bug: 23937688
Test: emerge-link update_engine; `mm`

Change-Id: Ic3a661e041a5709c116b6acd0d082a37b8bfe5df
diff --git a/platform_constants_android.cc b/platform_constants_android.cc
index 55ec974..be0329b 100644
--- a/platform_constants_android.cc
+++ b/platform_constants_android.cc
@@ -25,6 +25,8 @@
     "https://clients2.google.com/service/update2/brillo";
 const char kOmahaUpdaterID[] = "Brillo";
 const char kOmahaPlatformName[] = "Brillo";
+const char kUpdatePayloadPublicKeyPath[] =
+    "/etc/update_engine/update-payload-key.pub.pem";
 
 }  // namespace constants
 }  // namespace chromeos_update_engine