[test] Fix rootMicrodroid() to block till device is online. am: 1509817e72 am: 2703d302a2 am: 0d3dac971e

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

Change-Id: I2a41e273b745d4dddf6ec2af584b8f020674a452
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>