DPM.isUsbDataSignalingEnabled() callable on unmanaged device

Fix an issue in existing implementation where the API will
throw exception when called on a device without DO or PO: a
slight tweak of semantics such that when the API is called by
a regular app, return the device-wide policy regardless which
user the caller is from.

Bug: 190024751
Test: manual with modifed TestDPC on unmanaged device.
Change-Id: I227d01ec275bc0a074a3789bab194c76fc72b5b8