ZenModeHelper: fix cross deadlock with AudioService

Processing ZenModeHelper.setConfig() synchronously in
ZenModeConditions.onConditionChanged() can cause a cross deadlock between
ConditionProviders.mMutex and AudioService.mSettingsLock.

Add a method to set Zen mode config asynchronously.

Bug: 24528290.
Change-Id: I9c1701ca6bef084527821175d84002b612241995