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_chromeos.cc b/platform_constants_chromeos.cc
index 62c1720..2a6ce12 100644
--- a/platform_constants_chromeos.cc
+++ b/platform_constants_chromeos.cc
@@ -25,6 +25,8 @@
     "https://omaha.sandbox.google.com/service/update2";
 const char kOmahaUpdaterID[] = "ChromeOSUpdateEngine";
 const char kOmahaPlatformName[] = "Chrome OS";
+const char kUpdatePayloadPublicKeyPath[] =
+    "/usr/share/update_engine/update-payload-key.pub.pem";
 
 }  // namespace constants
 }  // namespace chromeos_update_engine