Adds identifier to TaskSnapshot

This identifier is introduced to differentiate two task snapshots in
AiAi. Currently, the timestamp (in millis) is chosen as the identifier.

Bug: 136563752
Test: atest TaskSnapshotPersisterLoaderTest
Change-Id: I249511d1e4bb34a93d5db08d2b1cea3b1ee5d034
diff --git a/proto/src/task_snapshot.proto b/proto/src/task_snapshot.proto
index 381d983..821db86 100644
--- a/proto/src/task_snapshot.proto
+++ b/proto/src/task_snapshot.proto
@@ -33,4 +33,5 @@
      bool is_translucent = 9;
      string top_activity_component = 10;
      float scale = 11;
- }
\ No newline at end of file
+     int64 id = 12;
+ }