Add 'test' prefix to device-side methods

This is required to be executed with the CTS infrastructure that we use.

Fixes: 146902684
Test: atest CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.MixedDeviceOwnerTest#testLockTask_defaultDialer
Change-Id: Ibc6a4428b074b9a7d533827eb2e9954d89f6df8a