libsnapshot: Move first-stage pid var constant into IDeviceInfo.

This will fail in vts_libsnapshot_test because the pid isn't set (and
even if it was, we don't want to kill any running snapuserd instances).
Rather than start an instance just for the sake of killing it, use
dependency injection instead.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: I1569f927e018319cf4008673a32d5526573d8d57