[test] Fix rootMicrodroid() to block till device is online. am: 1509817e72 am: 2703d302a2

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

Change-Id: Id52115c01f3c827eeda721cad5b64ab663d608c3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>