Sleep a little before waiting for reboot am: 4368a812af

Original change: https://android-review.googlesource.com/c/platform/tools/tradefederation/+/2184735

Change-Id: I31b693ef657857a19a8f3e1de5727233300bf8da
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>