Fix testAlwaysOnVpnAcrossReboot cts test

Test: run cts -m CtsDevicePolicyManagerTestCases -t
 com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testAlwaysOnVpnAcrossReboot

Root cause:
This is because the managed profile might not be initialized when the test real starts and device
reboots,so the created user might not be able to start after rebooting when the test continues to proceed

Solution:
wait until created user being initialized, then the test continues to run

Bug: 122489251

Change-Id: If605ddda2b4c070b5eaa49a1dc13f66f71b2d0bb
Signed-off-by: weijuncheng <weijuncheng@xiaomi.com>