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
9 files changed