[test] Fix rootMicrodroid() to block till device is online. am: 1509817e72

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Virtualization/+/2060047

Change-Id: I868fbdf6516df758f68237f591a1fdf78bc4a3ad
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>