Update StatsLogProcessor to handle BinaryPushStateChanged

Also changed StatsLog to call write() instead of the hard coded function
in StatsService.

Test: gts-tradefed run gts-dev --module GtsStatsdHostTestCases
Change-Id: I26171fa4cfc877e1e179b74ec8076d964aff8548
diff --git a/cmds/statsd/src/experiment_ids.proto b/cmds/statsd/src/experiment_ids.proto
new file mode 100644
index 0000000..c203631
--- /dev/null
+++ b/cmds/statsd/src/experiment_ids.proto
@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2020 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";
+
+package android.os.statsd;
+
+option java_package = "com.android.internal.os";
+option java_outer_classname = "ExperimentIdsProto";
+
+// StatsLogProcessor uses the proto to parse experiment ids from
+// BinaryPushStateChanged atoms. This needs to be in sync with
+// TrainExperimentIds in atoms.proto.
+message ExperimentIds {
+    repeated int64 experiment_id = 1;
+}