init: use protobuf for serialization of persistent properties

I probably should have done this from the start...  There's a shim to
convert my manually serialized format to protobuf, and since that has
not yet shipped, it'll be reverted in a short period of time.

Test: init unit tests
Test: upgrade from legacy and intermediate property formats successfully
Change-Id: Iad25f6c30d0b44d294230a53dd6876222d1c785b
diff --git a/init/persistent_properties.proto b/init/persistent_properties.proto
new file mode 100644
index 0000000..c8d2e3a
--- /dev/null
+++ b/init/persistent_properties.proto
@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+syntax = "proto2";
+option optimize_for = LITE_RUNTIME;
+
+message PersistentProperties {
+    message PersistentPropertyRecord {
+        optional string name = 1;
+        optional string value = 2;
+    }
+
+    repeated PersistentPropertyRecord properties = 1;
+}