Make sure silent mode changes with wait and check

After shell commands return, background service processes may still
execute the commands. The status changes may delay. So, added wait and
check to make sure silent mode changes are completed before host side
comparison.

Bug: 188577967
Test: atest
CtsCarHostTestCases: PowerPolicyHostTest#testPowerPolicySilentMode
Change-Id: Ia58352695632dbeb02b303aac371eab59e7f6dc9
2 files changed