Allow any apps to call DPM.isUsbDataSignalingEnabled()

This is to allow 3p apps to query the state of the policy so they
can show appropriate UX to the user in case the app's interaction
with the plugged-in USB devices is  disrupted by the admin policy.

Bug: 190024751
Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Change-Id: I829ff84256e0288b88c11add53da312ee2bc2558