ZenModeHelper: To pass android.media.cts.AudioManagerTest.

While CTS test is running to change the ringerMode from one to another various stage,
the ZenMode state change is initiating ringerMode internal state change at the same time.
And it results in CTS failure as it is not changing to the state that external is setting.

CRs-fixed: 808931
Change-Id: I1dc02e8934f3e3f34af709ce48ead5f6a4f8c84e