[DeviceConfig] add an API to delete a property

Currently there is only a shell command to delete a property.
Adding an API to do the same.

BUG: 184001163
Test: atest android.provider.DeviceConfigTest
Test: atest CtsDeviceConfigTestCases
Test: atest com.android.server.testables.TestableDeviceConfigTest
Change-Id: I43d7177c509501efd65e32ab3ad000d17d6ffb6c