releasetools: Add support for --override_timestamp. am: 9f884e6c55 am: 9ee7071553 am: d5de271e01 am: 839cdc66b1
am: 4b066f1dcc  -s ours

Change-Id: Ibb801d9c5171fc3aef3e2a0f3a046c636f69b356