Don't use temp file for public key.

If using public key from Omaha response, we wrote it to a temp file
which will be read back, this is unnecessary since we can keep it in
memory.

Test: update_engine_unittests
Change-Id: Ib9f7a9292b71b9d91a0b24c687cc989b79d3812b
diff --git a/update_attempter_android.cc b/update_attempter_android.cc
index a9033b7..a3974ab 100644
--- a/update_attempter_android.cc
+++ b/update_attempter_android.cc
@@ -396,8 +396,13 @@
         "Failed to read metadata and signature from " + metadata_filename);
   }
   fd->Close();
-  errorcode = payload_metadata.ValidateMetadataSignature(
-      metadata, "", base::FilePath(constants::kUpdatePayloadPublicKeyPath));
+
+  string public_key;
+  if (!utils::ReadFile(constants::kUpdatePayloadPublicKeyPath, &public_key)) {
+    return LogAndSetError(error, FROM_HERE, "Failed to read public key.");
+  }
+  errorcode =
+      payload_metadata.ValidateMetadataSignature(metadata, "", public_key);
   if (errorcode != ErrorCode::kSuccess) {
     return LogAndSetError(error,
                           FROM_HERE,