Use wait-for-device in vm_shell.sh

It is faster and more stable than waiting for 2 seconds.

Bug: 254934345
Test: Start a VM, run `vm/vm_shell.sh <cid>`
Change-Id: Ia126d568af8a2b2ef1497d7061ce5aa44d35f88c
diff --git a/vm/vm_shell.sh b/vm/vm_shell.sh
index 8217901..c0dd38f 100755
--- a/vm/vm_shell.sh
+++ b/vm/vm_shell.sh
@@ -27,7 +27,7 @@
     adb forward tcp:8000 vsock:${cid}:5555
     adb connect localhost:8000
     adb -s localhost:8000 root
-    sleep 2
+    adb -s localhost:8000 wait-for-device
     adb -s localhost:8000 shell
     exit 0
 }