Check initial state of ZenMode to be one of the valid values

Devices like Daydream headsets and Watches have default notification
conditions and suppressors and testZenMode was making some wrong assumptions
about the initial/default state of notifications on any Android devices.

Most conditions checked by current implementation of testZenMode are not
explicit requirements in CDD.

Changed the test to verify that the default value for ZenMode is a valid
value defined in API (one of allowed enum values).

Bug: 65739605
Test: `run -m CtsIncidentHostTestCases -t com.android.server.cts.NotificationTest#testZenMode`
      Test passes

Merged-In: Ie19d6b095d27c0789588e8a4354a79f13fe6c68e
Change-Id: I2a55979e4608e4db586e286e6e15220434c6492e